Methods and compositions for the specific inhibition of gene expression by double-stranded RNA

ABSTRACT

The invention is directed to compositions and methods for selectively reducing the expression of a gene product from a desired target gene in a cell, as well as for treating diseases caused by the expression of the gene. More particularly, the invention is directed to compositions that contain double stranded RNA (“dsRNA”), and methods for preparing them, that are capable of reducing the expression of target genes in eukaryotic cells. The dsRNA has a first oligonucleotide sequence that is between 25 and about 30 nucleotides in length and a second oligonucleotide sequence that anneals to the first sequence under biological conditions. In addition, a region of one of the sequences of the dsRNA having a sequence length of at least 19 nucleotides is sufficiently complementary to a nucleotide sequence of the RNA produced from the target gene to trigger the destruction of the target RNA by the RNAi machinery.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 12/138,215 filed 12 Jun. 2008, which in turn is a division ofU.S. patent application Ser. No. 11/079,906 filed 15 Mar. 2005, which inturn is related to and claims priority under 35 U.S.C. §119(e) to U.S.provisional patent application No. 60/553,487 filed 15 Mar. 2004. Eachapplication is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The present invention was made in part with Government support underGrant Numbers AI29329 and HL074704 awarded by the National Institute ofHealth. The Government has certain rights in this invention.

SEQUENCE SUBMISSION

The present application is being filed along with a Sequence Listing inelectronic format. The Sequence Listing is entitled1954552SequenceListing.txt, was created on 2 Jun. 2011 and is 26 kb insize. The information in the electronic format of the Sequence Listingis part of the present application and is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The publications and other materials used herein to illuminate thebackground of the invention, and in particular, cases to provideadditional details respecting the practice, are incorporated byreference, and for convenience are referenced in the following text byauthor and date and are listed alphabetically by author in the appendedbibliography.

The present invention pertains to compositions and methods forgene-specific inhibition of gene expression by double-strandedribonucleic acid (dsRNA) effector molecules. The compositions andmethods are useful in modulating gene expression in a variety ofapplications, including therapeutic, diagnostic, target validation, andgenomic discovery.

BACKGROUND OF THE INVENTION

Suppression of gene expression by double-stranded RNA (dsRNA) has beendemonstrated in a variety of systems including plants(post-transcriptional gene suppression) (Napoli et al., 1990), fungi(quelling) (Romano and Marcino, 1992), and nematodes (RNA interference)(Fire et al., 1998). Early attempts to similarly suppress geneexpression using long dsRNAs in mammalian systems failed due toactivation of interferon pathways that do not exist in lower organisms.Interferon responses are triggered by dsRNAs (Stark et al., 1998). Inparticular, the protein kinase PKR is activated by dsRNAs of greaterthan 30 bp long (Manche et al., 1992) and results in phosphorylation oftranslation initiation factor eIF2α which leads to arrest of proteinsynthesis and activation of 2′S′-oligoadenylate synthetase (2′-5′-OAS),which leads to RNA degradation (Minks et al., 1979).

In Drosophila cells and cell extracts, dsRNAs of 150 bp length orgreater were seen to induce RNA interference while shorter dsRNAs wereineffective (Tuschl et al., 1999). Long double-stranded RNA, however, isnot the active effecter molecule; long dsRNAs are degraded by an RNaseIII class enzyme called Dicer (Bernstein et al., 2001) into very short21-23 bp duplexes that have 2-base 3′-overhangs (Zamore et al., 2000).These short RNA duplexes, called siRNAs, direct the RNAi response invivo and transfection of short chemically synthesized siRNA duplexes ofthis design permits use of RNAi methods to suppress gene expression inmammalian cells without triggering unwanted interferon responses(Elbashir et al., 2001a). The antisense strand of the siRNA duplexserves as a sequence-specific guide that directs activity of anendoribonuclease function in the RNA induced silencing complex (RISC) todegrade target mRNA (Martinez et al., 2002).

In studying the size limits for RNAi in Drosophila embryo extracts invitro, a lower threshold of around 38 bp double-stranded RNA wasestablished for activation of RNA interference using exogenouslysupplied double-stranded RNA and duplexes of 36, 30, and 29 bp length(Elbashir et al., 2001b). The short 30-base RNAs were not cleaved intoactive 21-23-base siRNAs and therefore were deemed inactive for use inRNAi (Elbashir et al., 2001b). Continuing to work in the Drosophilaembryo extract system, the same group later carefully mapped thestructural features needed for short chemically synthesized RNA duplexesto function as siRNAs in RNAi pathways. RNA duplexes of 21-bp lengthwith 2-base 3′-overhangs were most effective, duplexes of 20, 22, and23-bp length had slightly decreased potency but did result in RNAimediated mRNA degradation, and 24 and 25-bp duplexes were inactive(Elbashir et al., 2001c).

Some of the conclusions of these earlier studies may be specific to theDrosophila system employed. Other investigators established that longersiRNAs can work in human cells. However, duplexes in the 21-23-bp rangehave been shown to be more active and have become the accepted design(Caplen et al., 2001). Essentially, chemically synthesized duplex RNAsthat mimicked the natural products that result from Dicer degradation oflong duplex RNAs were identified to be the preferred compound for use inRNAi. Approaching this problem from the opposite direction,investigators studying size limits for RNAi in Caenorhabditis elegansfound that although a microinjected 26-bp RNA duplex could function tosuppress gene expression, it required a 250-fold increase inconcentration compared with an 81-bp duplex (Parrish et al., 2000).

Despite the attention given to RNAi research recently, the field isstill in the early stages of development. Not all siRNA molecules arecapable of targeting the destruction of their complementary RNAs in acell. As a result, complex sets of rules have been developed fordesigning RNAi molecules that will be effective. Those having skill inthe art expect to test multiple siRNA molecules to find functionalcompositions. (Ji et al., 2003) Some artisans pool several siRNApreparations together to increase the chance of obtaining silencing in asingle study. (Ji et al., 2003) Such pools typically contain 20 nM of amixture of siRNA oligonucleotide duplexes or more (Ji et al., 2003),despite the fact that a siRNA molecule can work at concentrations of 1nM or less (Holen et al., 2002). This technique can lead to artifactscaused by interactions of the siRNA sequences with other cellular RNAs(“off target effects”). (Scherer et al., 2003) Off target effects canoccur when the RNAi oligonucleotides have homology to unintended targetsor when the RISC complex incorporates the unintended strand from andRNAi duplex. (Scherer et al., 2003) Generally, these effects tend to bemore pronounced when higher concentrations of RNAi duplexes are used.(Scherer et al., 2003)

In addition, the duration of the effect of an effective RNAi treatmentis limited to about 4 days (Holen et al., 2002). Thus, researchers mustcarry out siRNA experiments within 2-3 days of transfection with ansiRNA duplex or work with plasmid or viral expression vectors to obtainlonger term silencing.

The invention provides compositions useful in RNAi for inhibiting geneexpression and provides methods for their use. In addition, theinvention provides RNAi compositions and methods designed to maximizepotency, potentially increase duration of action and ease site selectioncriteria, while minimizing “off target effects.” These and otheradvantages of the invention, as well as additional inventive features,will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to compositions and methods forselectively reducing the expression of a gene product from a desiredtarget gene in a eukaryotic cell, as well as for treating diseasescaused by the expression of the gene. More particularly, the inventionis directed to compositions that contain double stranded RNA (“dsRNA”),and methods for preparing them, that are capable of reducing theexpression of target genes in eukaryotic cells.

Thus, in a first aspect, the present invention provides novelcompositions for RNA interference (RNAi). The compositions comprisedsRNA which is a precursor molecule, i.e., the dsRNA of the presentinvention is processed in vivo to produce an active siRNA. The dsRNA isprocessed by Dicer to an active siRNA which is incorporated into theRISC complex for RNA interference of a target gene. The precursormolecule is also termed a precursor RNAi molecule herein.

In one embodiment, the dsRNA, i.e., the precursor RNAi molecule, has alength sufficient such that it is processed by Dicer to produce ansiRNA. According to this embodiment, the dsRNA comprises a firstoligonucleotide sequence (also termed the sense strand) that is between26 and about 30 nucleotides in length and a second oligonucleotidesequence (also termed the antisense strand) that anneals to the firstsequence under biological conditions, such as the conditions found inthe cytoplasm of a cell. In addition, a region of one of the sequences,particularly of the antisense strand, of the dsRNA has a sequence lengthof at least 19 nucleotides, for example, from about 19 to about 23nucleotides, such as 21 nucleotides that are sufficiently complementaryto a nucleotide sequence of the RNA produced from the target gene totrigger an RNAi response.

In a second embodiment, the dsRNA, i.e., the precursor RNAi molecule,has several properties which enhance its processing by Dicer. Accordingto this embodiment, the dsRNA dsRNA has a length sufficient such that itis processed by Dicer to produce an siRNA and at least one of thefollowing properties: (ii) the dsRNA is asymmetric, e.g., has a 3′overhang on the antisense strand and (ii) the dsRNA has a modified 3′end on the sense strand to direct orientation of Dicer binding andprocessing of the dsRNA to an active siRNA. According to thisembodiment, the sense strand comprises 22-28 nucleotides and theantisense strand comprises 24-30 nucleotides. In one embodiment, thedsRNA has an overhang on the 3′ end of the antisense strand. In anotherembodiment, the sense strand is modified for Dicer binding andprocessing by suitable modifiers located at the 3′ end of the sensestrand. Suitable modifiers include nucleotides such asdeoxyribonucleotides, acyclonucleotides and the like and stericallyhindered molecules, such as fluorescent molecules and the like. Whennucleotide modifiers are used, they replace ribonucleotides in the dsRNAsuch that the length of the dsRNA does not change. In anotherembodiment, the dsRNA has an overhang on the 3′ end of the antisensestrand and the sense strand is modified for Dicer processing. In anotherembodiment, the 5′ end of the sense strand has a phosphate. The senseand antisense strands anneal under biological conditions, such as theconditions found in the cytoplasm of a cell. In addition, a region ofone of the sequences, particularly of the antisense strand, of the dsRNAhas a sequence length of at least 19 nucleotides, wherein thesenucleotides are in the 21-nucleotide region adjacent to the 3′ end ofthe antisense strand and are sufficiently complementary to a nucleotidesequence of the RNA produced from the target gene. Further in accordancewith this embodiment, the dsRNA, i.e., the precursor RNAi molecule, mayalso have one or more of the following additional properties: (a) theantisense strand has a right shift from the typical 21 mer (i.e., theantisense strand includes nucleotides on the right side of the moleculewhen compared to the typical 21 mer), (b) the strands may not becompletely complementary, i.e., the strands may contain simple mismatchpairings and (c) base modifications such as locked nucleic acid(s) maybe included in the 5′ end of the sense strand.

In a third embodiment, the dsRNA, i.e., the precursor RNAi molecule, hasseveral properties which enhance its processing by Dicer. According tothis embodiment, the dsRNA has a length sufficient such that it isprocessed by Dicer to produce an siRNA and at least one of the followingproperties: (i) the dsRNA is asymmetric, e.g., has a 3′ overhang on thesense strand and (ii) the dsRNA has a modified 3′ end on the antisensestrand to direct orientation of Dicer binding and processing of thedsRNA to an active siRNA. According to this embodiment, the sense strandcomprises 24-30 nucleotides and the antisense strand comprises 22-28nucleotides. In one embodiment, the dsRNA has an overhang on the 3′ endof the sense strand. In another embodiment, the antisense strand ismodified for Dicer binding and processing by suitable modifiers locatedat the 3′ end of the antisense strand. Suitable modifiers includenucleotides such as deoxyribonucleotides, acyclonucleotides and the likeand sterically hindered molecules, such as fluorescent molecules and thelike. When nucleotide modifiers are used, they replace ribonucleotidesin the dsRNA such that the length of the dsRNA does not change. Inanother embodiment, the dsRNA has an overhang on the 3′ end of the sensestrand and the antisense strand is modified for Dicer processing. In oneembodiment, the antisense strand has a 5′ phosphate. The sense andantisense strands anneal under biological conditions, such as theconditions found in the cytoplasm of a cell. In addition, a region ofone of the sequences, particularly of the antisense strand, of the dsRNAhas a sequence length of at least 19 nucleotides, wherein thesenucleotides are adjacent to the 3′ end of antisense strand and aresufficiently complementary to a nucleotide sequence of the RNA producedfrom the target gene. Further in accordance with this embodiment, thedsRNA, i.e., the precursor RNAi molecule, may also have one or more ofthe following additional properties: (a) the antisense strand has a leftshift from the typical 21 mer (i.e., the antisense strand includesnucleotides on the left side of the molecule when compared to thetypical 21 mer) and (b) the strands may not be completely complementary,i.e., the strands may contain simple mismatch pairings.

In a second aspect, the present invention provides a method for making adsRNA, i.e., a precursor RNAi molecule, that has enhanced processing byDicer. According to this method an antisense strand siRNA having alength of at least 19 nucleotides is selected for a given target geneusing conventional techniques and algorithms. In one embodiment, theantisense siRNA is modified to include 5-11 ribonucleotides on the 5′end to give a length of 24-30 nucleotides. When the antisense strand hasa length of 21 nucleotides, then 3-9 nucleotides, or 4-7 nucleotides or6 nucleotides are added on the 5′ end. Although the addedribonucleotides may be complementary to the target gene sequence, fullcomplementarity between the target sequence and the antisense siRNA isnot required. That is, the resultant antisense siRNA is sufficientlycomplementary with the target sequence. A sense strand is then producedthat has 22-28 nucleotides. The sense strand is substantiallycomplementary with the antisense strand to anneal to the antisensestrand under biological conditions. In one embodiment, the sense strandis synthesized to contain a modified 3′ end to direct Dicer processingof the antisense strand. In another embodiment, the antisense strand ofthe dsRNA has a 3′ overhang. In a further embodiment, the sense strandis synthesized to contain a modified 3′ end for Dicer binding andprocessing and the antisense strand of the dsRNA has a 3′ overhang.

In a second embodiment of this method, the antisense siRNA is modifiedto include 1-9 ribonucleotides on the 5′ end to give a length of 22-28nucleotides. When the antisense strand has a length of 21 nucleotides,then 1-7 ribonucleotides, or 2-5 ribonucleotides and or 4ribonucleotides are added on the 3′ end. The added ribonucleotides mayhave any sequence. Although the added ribonucleotides may becomplementary to the target gene sequence, full complementarity betweenthe target sequence and the antisense siRNA is not required. That is,the resultant antisense siRNA is sufficiently complementary with thetarget sequence. A sense strand is then produced that has 24-30nucleotides. The sense strand is substantially complementary with theantisense strand to anneal to the antisense strand under biologicalconditions. In one embodiment, the antisense strand is synthesized tocontain a modified 3′ end to direct Dicer processing. In anotherembodiment, the sense strand of the dsRNA has a 3′ overhang. In afurther embodiment, the antisense strand is synthesized to contain amodified 3′ end for Dicer binding and processing and the sense strand ofthe dsRNA has a 3′ overhang.

In a third aspect, the present invention provides pharmaceuticalcompositions containing the disclosed dsRNA compositions.

In a fourth aspect, the present invention provides methods forselectively reducing the expression of a gene product from a desiredtarget gene in a cell, as well as for treating diseases caused by theexpression of the gene. In one embodiment, the method involvesintroducing into the environment of a cell an amount of a dsRNA of thepresent invention such that a sufficient portion of the dsRNA can enterthe cytoplasm of the cell to cause a reduction in the expression of thetarget gene.

The compositions and methods have an unanticipated level of potency ofthe RNAi effect. Although the invention is not intended to be limited bythe underlying theory on which it is believed to operate, it is thoughtthat this level of potency and duration of action are caused by the factthe dsRNA serves as a substrate for Dicer which appears to facilitateincorporation of one sequence from the dsRNA into the RISC complex thatis directly responsible for destruction of the RNA from the target gene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F show that 27 mer dsRNAs are more potent effectors of RNAithan a 21+2 siRNA. EGFP expression levels were determined aftercotransfection of HEK293 cells with a fixed amount of EGFP expressionplasmid and varying concentrations of dsRNAs. FIGS. 1A-1C: Transfectionswere performed using (FIG. 1A) 50 nM, (FIG. 1B) 200 pM and (FIG. 1C) 50pM of the indicated dsRNAs. FIG. 1D: Dose-response testing of longerdsRNAs. Transfections were performed with the indicated concentrationsof dsRNA. FIG. 1E: Depicts in vitro Dicer reactions with the same longerRNAs. Concentrations and conditions were as described in the Examples.FIG. 1F: Dose-response curve of dsRNAs transfected into NIH3T3 cellsthat stably express EGFP. Each graph point represents the average (withs.d.) of three independent measurements.

FIGS. 2A-2E show that Dicer processing correlates with RNAi activity.FIG. 2A: Cleavage of dsRNAs by recombinant Dicer. Each RNA duplex wasincubated in the presence or absence of recombinant human Dicer for 24h, separated using nondenaturing PAGE and visualized (see Examples).FIG. 2B: RNA duplexes used in this test. Oligos were conjugated with6FAM at the 5′ ends, the 3′ ends or both as shown by the circles. Topand bottom lines indicate sense and antisense strands in duplexconfiguration, with the sense in a 5′-to-3′ orientation (left to right)and the antisense in a 3′-to-5′ orientation (left to right). FIG. 2C:6FAM end-modification affects in vitro Dicer activity. RNA duplexes wereincubated with 0.5 units of recombinant human Dicer for 8 h and theproducts resolved on a 7.5% nondenaturing polyacrylamide gel. The RNAswere visualized by ethidium bromide staining FIG. 2D: 6FAM modificationaffects RNAi activity. RNA duplexes at 200 pM were cotransfected withthe EGFP expression plasmid and assayed at 24 h for EGFP fluorescence asdescribed. Reported values for EGFP expression represent the average oftwo independent experiments. The relative levels of fluorescence werenormalized to those for luciferase. FIG. 2E: 27 mer duplex RNAs areprocessed to 21 mers in vivo. Total RNA was prepared from cellstransfected with duplex 3 and duplex 5 at 10 nM. RNA was hybridized witha 21 mer ³²P-labeled oligonucleotide probe. The hybridized samples wereseparated by nondenaturing PAGE and visualized by autoradiography. Sizemarkers are ³²P end labeled 21 mer and 27 mer RNA duplexes.

FIG. 3A-3B show RNAi activity of various 21+2 siRNAs. FIG. 3A: Sevenpossible 21+2 siRNAs predicted from Dicing the 27 mer dsRNA were testedindividually or as a pool in co-transfection assays with the EGFPreporter construct in HEK293 cells. Each graph depicts the average ofduplicate experiments. FIG. 3B: Comparison of in vitro Diced 27 merdsRNA versus intact 27 mer dsRNA for RNAi. The respective RNAs wereco-transfected as in FIG. 4A at the indicated concentrations of dsRNAs.For the Diced products, a 1 μM 27 mer dsRNA was incubated in Dicerreaction buffer without (column 3) or with Dicer (column 4) at 37° C.for 12 hours. The mixtures were diluted in water and used directly forco-transfection with the EGFP reporter. To control for possibleartifacts of residual Dicer in the diluted mixes, the samples in column4 were phenol extracted and ethanol precipitated prior to transfection(column 5).

FIGS. 4A and 4B show ESI mass spectra of the 27 mer duplex EGFPS1 27+0before (FIG. 4A) and after (FIG. 4B) incubation with Dicer. Duplexesseparate into single strands and the measured mass of each strand isindicated. Dicer digestion is performed in the presence of high salt andsome “shadow” peaks represent +1 or +2 Na species.

FIGS. 5A-5D show features of 27 mer dsRNA in RNAi. FIG. 5A: Enhancedduration of RNAi by 27 mer dsRNAs. Levels of EGFP were determined aftertransfection of 5 nM of a 21+2 siRNA or the 27 mer dsRNA into NIH3T3cells stably expressing EGFP. Graphic representation of EGFP silencingmediated by a 21+2 siRNA as compared to the 27 mer dsRNA. Duplicatesamples were taken on the indicated days and EGFP expression wasdetermined by fluorometry. FIG. 5B: 27 mer dsRNAs, targeting sitesrefractory to 21 mer siRNAs, can elicit RNAi. The dsRNAs weretransfected along with the EGFP reporter construct, and EGFP expressionwas determined (Methods). Column 1, mock; column 2, 21+2 siRNA targetingEGFPS2; column 3, 27 mer dsRNA targeting EGFPS2; column 4, 21+2 siRNAtargeting EGFPS3; column 5, 27 mer dsRNA targeting EGFPS3. FIGS. 5C and5D: Comparison of 21 mer siRNA and 27 mer dsRNA in downregulation ofendogenous transcripts. RNAs for a 21+2 siRNA and 27+0 dsRNA weredesigned to target sites in the hnRNP H mRNA (FIG. 5C) or La mRNA (FIG.5D). HnRNP H knockdown was assayed by western blot and La knockdown bynorthern blot analyses. The dsRNAs were used at the indicatedconcentrations. β-Actin was used as an internal specificity and loadingstandard in both experiments.

FIG. 6 shows sequence specificity of Dicer substrate 27 mer dsRNAs. Thevarious 27 mer dsRNAs were co-transfected at the indicatedconcentrations with the EGFP expression plasmid into HEK93 cells andassayed for EGFP fluorescence.

FIGS. 7A-7D show that siRNAs and Dicer substrate dsRNAs do not induceinterferons or activate PKR or generate specific “off target effects.”FIGS. 7A and 7B: Interferon alpha (FIG. 7A) and interferon beta (FIG.7B) assays: column 1, positive control for IFN induction (Kim et al.,2004); column 2, no RNA; column 3, chemically synthesized 21+2 siRNA;column 4, chemically synthesized 27+0 dsRNA. FIG. 7C: PKR activationassay. The lond dsRNA used for PKR activation (Kim et al., 2004) and thein vitro PKR activation assay (Manche et al, 1992) have been previouslydescribed. Duplex RNAs were transfected as indicated. FIG. 7D: Summaryof microarray analysis.

FIGS. 8A-8B show ESI mass spectra of the 27 mer duplex EGFPS1 27+0 Lbefore (FIG. 8A) and after (FIG. 8B) incubation with Dicer. Duplexesseparate into single strands and the measured mass of each strand isindicated.

FIGS. 8C-8D show ESI mass spectra of the 27 mer duplex EGFPS1 27/25 Lbefore (FIG. 8C) and after (FIG. 8D) incubation with Dicer. Duplexesseparate into single strands and the measured mass of each strand isindicated.

FIGS. 9A-9B show ESI mass spectra of the 27 mer duplex EGFPS1 27+0 Rbefore (FIG. 9A) and after (FIG. 9B) incubation with Dicer. Duplexesseparate into single strands and the measured mass of each strand isindicated.

FIGS. 9C-9D show ESI mass spectra of the 27 mer duplex EGFPS1 25/27 Rbefore (FIG. 9C) and after (FIG. 9B) incubation with Dicer. Duplexesseparate into single strands and the measured mass of each strand isindicated.

FIGS. 10A-10B show that duplexes designed to enhance Dicer processingare potent effectors of RNAi. EGFP expression levels were determinedafter cotransfection of HEK293 cells with a fixed amount of EGFPexpression plasmid and varying concentrations of dsRNAs. FIG. 10A:Compares the potency of the duplexes EGFPS2-21+2, EGFPS2-27+0,EGFPS2-27/25 L and EGFPS2-25/27 R. FIG. 10B: Compares the potency of theduplexes EGFPS1-27/25 L and EGFPS1-25/27 R.

FIG. 11 is an illustration showing two embodiments of the presentinvention with respect to the target sequence and the relationshipbetween the target sequence and each embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to compositions that contain doublestranded RNA (“dsRNA”), and methods for preparing them, that are capableof reducing the expression of target genes in eukaryotic cells. One ofthe strands of the dsRNA contains a region of nucleotide sequence thathas a length that ranges from about 19 to about 30 nucleotides that candirect the destruction of the RNA transcribed from the target gene.

In a first aspect, the present invention provides novel compositions forRNA interference (RNAi). The compositions comprise dsRNA which is aprecursor molecule, i.e., the dsRNA of the present invention isprocessed in vivo to produce an active siRNA. The dsRNA is processed byDicer to an active siRNA which is incorporated into the RISC complex.The precursor molecule is also termed a precursor RNAi molecule herein.As used herein, the term active siRNA refers to a double strandednucleic acid in which each strand comprises RNA, RNA analog(s) or RNAand DNA. The siRNA comprises between 19 and 23 nucleotides or comprises21 nucleotides. The active siRNA has 2 bp overhangs on the 3′ ends ofeach strand such that the duplex region in the siRNA comprises 17-21nucleotides, or 19 nucleotides. Typically, the antisense strand of thesiRNA is sufficiently complementary with the target sequence of thetarget gene.

The phrase “duplex region” refers to the region in two complementary orsubstantially complementary oligonucleotides that form base pairs withone another, either by Watson-Crick base pairing or any other mannerthat allows for a duplex between oligonucleotide strands that arecomplementary or substantially complementary. For example, anoligonucleotide strand having 21 nucleotide units can base pair withanother oligonucleotide of 21 nucleotide units, yet only 19 bases oneach strand are complementary or substantially complementary, such thatthe “duplex region” consists of 19 base pairs. The remaining base pairsmay, for example, exist as 5′ and 3′ overhangs. Further, within theduplex region, 100% complementarity is not required; substantialcomplementarity is allowable within a duplex region. Substantialcomplementarity refers to complementarity between the strands such thatthey are capable of annealing under biological conditions. Techniques toempirically determine if two strands are capable of annealing underbiological conditions are well know in the art. Alternatively, twostrands can be synthesized and added together under biologicalconditions to determine if they anneal to one another.

As used herein, an siRNA having a sequence “sufficiently complementary”to a target mRNA sequence means that the siRNA has a sequence sufficientto trigger the destruction of the target mRNA by the RNAi machinery(e.g., the RISC complex) or process. The siRNA molecule can be designedsuch that every residue of the antisense strand is complementary to aresidue in the target molecule. Alternatively, substitutions can be madewithin the molecule to increase stability and/or enhance processingactivity of said molecule. Substitutions can be made within the strandor can be made to residues at the ends of the strand.

In one embodiment of the first aspect of the present invention, thedsRNA, i.e., the precursor RNAi molecule, has a length sufficient suchthat it is processed by Dicer to produce an siRNA. According to thisembodiment, a suitable dsRNA contains one oligonucleotide sequence, afirst sequence, that is at least 25 nucleotides in length and no longerthan about 30 nucleotides. This sequence of RNA can be between about 26and 29 nucleotides in length. This sequence can be about 27 or 28nucleotides in length or 27 nucleotides in length. The second sequenceof the dsRNA can be any sequence that anneals to the first sequenceunder biological conditions, such as within the cytoplasm of aeukaryotic cell. Generally, the second oligonucleotide sequence willhave at least 19 complementary base pairs with the first oligonucleotidesequence, more typically the second oligonucleotides sequence will haveabout 21 or more complementary base pairs, or about 25 or morecomplementary base pairs with the first oligonucleotide sequence. In oneembodiment, the second sequence is the same length as the firstsequence, and the dsRNA is blunt ended. In another embodiment, the endsof the dsRNA have overhangs.

In certain aspects of this first embodiment, the first and secondoligonucleotide sequences of the dsRNA exist on separate oligonucleotidestrands that can be and typically are chemically synthesized. In someembodiments, both strands are between 26 and 30 nucleotides in length.In other embodiments, both strands are between 25 and 30 nucleotides inlength. In one embodiment, both strands are 27 nucleotides in length,are completely complementary and have blunt ends. The dsRNA can be froma single RNA oligonucleotide that undergoes intramolecular annealing or,more typically, the first and second sequences exist on separate RNAoligonucleotides. In one embodiment, one or both oligonucleotide strandsare capable of serving as a substrate for Dicer. In other embodiments,at least one modification is present that promotes Dicer to bind to thedouble-stranded RNA structure in an orientation that maximizes thedouble-stranded RNA structure's effectiveness in inhibiting geneexpression. The dsRNA can contain one or more deoxyribonucleic acid(DNA) base substitutions.

Suitable dsRNA compositions that contain two separate oligonucleotidescan be chemically linked outside their annealing region by chemicallinking groups. Many suitable chemical linking groups are known in theart and can be used. Suitable groups will not block Dicer activity onthe dsRNA and will not interfere with the directed destruction of theRNA transcribed from the target gene. Alternatively, the two separateoligonucleotides can be linked by a third oligonucleotide such that ahairpin structure is produced upon annealing of the two oligonucleotidesmaking up the dsRNA composition. The hairpin structure will not blockDicer activity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene.

The first and second oligonucleotides are not required to be completelycomplementary. In fact, in one embodiment, the 3′-terminus of the sensestrand contains one or more mismatches. In one aspect, about twomismatches are incorporated at the 3′ terminus of the sense strand. Inanother embodiment, the dsRNA of the invention is a double stranded RNAmolecule containing two RNA oligonucleotides each of which is 27nucleotides in length and, when annealed to each other, have blunt endsand a two nucleotide mismatch on the 3′-terminus of the sense strand(the 5′-terminus of the antisense strand). The use of mismatches ordecreased thermodynamic stability (specifically at the3′-sense/5′-antisense position) has been proposed to facilitate or favorentry of the antisense strand into RISC (Schwarz et al., 2003; Khvorovaet al., 2003), presumably by affecting some rate-limiting unwindingsteps that occur with entry of the siRNA into RISC. Thus, terminal basecomposition has been included in design algorithms for selecting active21 mer siRNA duplexes (Ui-Tei et al., 2004; Reynolds et al., 2004). WithDicer cleavage of the dsRNA of this embodiment, the small end-terminalsequence which contains the mismatches will either be left unpaired withthe antisense strand (become part of a 3′-overhang) or be cleavedentirely off the final 21-mer siRNA. These “mismatches”, therefore, donot persist as mismatches in the final RNA component of RISC. It wassurprising to find that base mismatches or destabilization of segmentsat the 3′-end of the sense strand of Dicer substrate improved thepotency of synthetic duplexes in RNAi, presumably by facilitatingprocessing by Dicer.

It has been found empirically that these longer dsRNA species of from 25to about 30 nucleotides give unexpectedly effective results in terms ofpotency and duration of action. Without wishing to be bound by theunderlying theory of the invention, it is thought that the longer dsRNAspecies serve as a substrate for the enzyme Dicer in the cytoplasm of acell. In addition to cleaving the dsRNA of the invention into shortersegments, Dicer is thought to facilitate the incorporation of asingle-stranded cleavage product derived from the cleaved dsRNA into theRISC complex that is responsible for the destruction of the cytoplasmicRNA derived from the target gene. The studies described herein haveshown that the cleavability of a dsRNA species by Dicer corresponds withincreased potency and duration of action of the dsRNA species.

In a second embodiment of the first aspect of the present invention, thedsRNA, i.e., the precursor RNAi molecule, has several properties whichenhance its processing by Dicer. According to this embodiment, the dsRNAhas a length sufficient such that it is processed by Dicer to produce anactive siRNA and at least one of the following properties: (i) the dsRNAis asymmetric, e.g., has a 3′ overhang on the antisense strand and (ii)the dsRNA has a modified 3′ end on the sense strand to directorientation of Dicer binding and processing of the dsRNA to an activesiRNA. According to this embodiment, the longest strand in the dsRNAcomprises 24-30 nucleotides. In one embodiment, the dsRNA is asymmetricsuch that the sense strand comprises 22-28 nucleotides and the antisensestrand comprises 24-30 nucleotides. Thus, the resulting dsRNA has anoverhang on the 3′ end of the antisense strand. The overhang is 1-3nucleotides, for example 2 nucleotides. The sense strand may also have a5′ phosphate. In another embodiment, at least one of the nucleotidesequences of the double stranded RNA composition comprises a nucleotideoverhang of between about one and about four nucleotides in length.

In another embodiment, the sense strand is modified for Dicer processingby suitable modifiers located at the 3′ end of the sense strand, i.e.,the dsRNA is designed to direct orientation of Dicer binding andprocessing. Suitable modifiers include nucleotides such asdeoxyribonucleotides, dideoxyribonucleotides, acyclonucleotides and thelike and sterically hindered molecules, such as fluorescent moleculesand the like. Acyclonucleotides substitute a 2-hydroxyethoxymethyl groupfor the 2′-deoxyribofuranosyl sugar normally present in dNMPs. Othernucleotides modifiers could include 3′-deoxyadenosine (cordycepin),3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxyinosine (ddI),2′,3′-dideoxy-3′-thiacytidine (3TC),2′,3′-didehydro-2′,3′-dideoxythymidine (d4T) and the monophosphatenucleotides of 3′-azido-3′-deoxythymidine (AZT),2′,3′-dideoxy-3′-thiacytidine (3TC) and2′,3′-didehydro-2′,3′-dideoxythymidine (d4T). In one embodiment,deoxynucleotides are used as the modifiers. When nucleotide modifiersare utilized, 1-3 nucleotide modifiers, or 2 nucleotide modifiers aresubstituted for the ribonucleotides on the 3′ end of the sense strand.When sterically hindered molecules are utilized, they are attached tothe ribonucleotide at the 3′ end of the antisense strand. Thus, thelength of the strand does not change with the incorporation of themodifiers. In another embodiment, the invention contemplatessubstituting two DNA bases in the dsRNA to direct the orientation ofDicer processing of the antisense strand. In a further embodiment of thepresent invention, two terminal DNA bases are substituted for tworibonucleotides on the 3′-end of the sense strand forming a blunt end ofthe duplex on the 3′ end of the sense strand and the 5′ end of theantisense strand, and a two-nucleotide RNA overhang is located on the3′-end of the antisense strand. This is an asymmetric composition withDNA on the blunt end and RNA bases on the overhanging end.

The sense and antisense strands anneal under biological conditions, suchas the conditions found in the cytoplasm of a cell. In addition, aregion of one of the sequences, particularly of the antisense strand, ofthe dsRNA has a sequence length of at least 19 nucleotides, whereinthese nucleotides are in the 21-nucleotide region adjacent to the 3′ endof the antisense strand and are sufficiently complementary to anucleotide sequence of the RNA produced from the target gene.

Further in accordance with this embodiment, the dsRNA, i.e., theprecursor RNAi molecule, may also have one or more of the followingadditional properties: (a) the antisense strand has a right shift fromthe typical 21 mer, (b) the strands may not be completely complementary,i.e., the strands may contain simple mismatch pairings and (c) basemodifications such as locked nucleic acid(s) may be included in the 5′end of the sense strand. A “typical” 21 mer siRNA is designed usingconventional techniques. In one technique, a variety of sites arecommonly tested in parallel or pools containing several distinct siRNAduplexes specific to the same target with the hope that one of thereagents will be effective (Ji et al., 2003). Other techniques usedesign rules and algorithms to increase the likelihood of obtainingactive RNAi effector molecules (Schwarz et al., 2003; Khvorova et al.,2003; Ui-Tei et al., 2004; Reynolds et al., 2004; Krol et al., 2004;Yuan et al., 2004; Boese et al., 2005). High throughput selection ofsiRNA has also been developed (U.S. published patent application No.2005/0042641 A1, incorporated herein by reference). Potential targetsites can also be analyzed by secondary structure predictions (Heale etal., 2005). This 21 mer is then used to design a right shift to include3-9 additional nucleotides on the 5′ end of the 21 mer. The sequence ofthese additional nucleotides may have any sequence. In one embodiment,the added ribonucleotides are based on the sequence of the target gene.Even in this embodiment, full complementarity between the targetsequence and the antisense siRNA is not required.

The first and second oligonucleotides are not required to be completelycomplementary. They only need to be substantially complementary toanneal under biological conditions and to provide a substrate for Dicerthat produces an siRNA sufficiently complementary to the targetsequence. Locked nucleic acids, or LNA's, are well known to a skilledartisan (Elman et al., 2005; Kurreck et al., 2002; Crinelli et al.,2002; Braasch and Corey, 2001; Bondensgaard et al., 2000; Wahlestedt etal., 2000). In one embodiment, an LNA is incorporated at the 5′ terminusof the sense strand. In another embodiment, an LNA is incorporated atthe 5′ terminus of the sense strand in duplexes designed to include a 3′overhang on the antisense strand.

In one embodiment, the dsRNA has an asymmetric structure, with the sensestrand having a 25-base pair length, and the antisense strand having a27-base pair length with a 2 base 3′-overhang. In another embodiment,this dsRNA having an asymmetric structure further contains 2deoxynucleotides at the 3′ end of the sense strand in place of two ofthe ribonucleotides.

Suitable dsRNA compositions that contain two separate oligonucleotidescan be linked by a third structure. The third structure will not blockDicer activity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene. In oneembodiment, the third structure may be a chemical linking group. Manysuitable chemical linking groups are known in the art and can be used.Alternatively, the third structure may be an oligonucleotide that linksthe two oligonucleotides of the dsRNA is a manner such that a hairpinstructure is produced upon annealing of the two oligonucleotides makingup the dsRNA composition. The hairpin structure will not block Diceractivity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene.

In a third embodiment of the first aspect of the present invention, thedsRNA, i.e., the precursor RNAi molecule, has several properties whichenhances its processing by Dicer. According to this embodiment, thedsRNA has a length sufficient such that it is processed by Dicer toproduce an siRNA and at least one of the following properties: (i) thedsRNA is asymmetric, e.g., has a 3′ overhang on the sense strand and(ii) the dsRNA has a modified 3′ end on the antisense strand to directorientation of Dicer binding and processing of the dsRNA to an activesiRNA. According to this embodiment, the longest strand in the dsRNAcomprises 24-30 nucleotides. In one embodiment, the sense strandcomprises 24-30 nucleotides and the antisense strand comprises 22-28nucleotides. Thus, the resulting dsRNA has an overhang on the 3′ end ofthe sense strand. The overhang is 1-3 nucleotides, such as 2nucleotides. The antisense strand may also have a 5′ phosphate.

In another embodiment, the antisense strand is modified for Dicerprocessing by suitable modifiers located at the 3′ end of the antisensestrand, i.e., the dsRNA is designed to direct orientation of Dicerbinding and processing. Suitable modifiers include nucleotides such asdeoxyribonucleotides, dideoxyribonucleotides, acyclonucleotides and thelike and sterically hindered molecules, such as fluorescent moleculesand the like. Acyclonucleotides substitute a 2-hydroxyethoxymethyl groupfor the 2′-deoxyribofuranosyl sugar normally present in dNMPs. Othernucleotide modifiers could include 3′-deoxyadenosine (cordycepin),3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxyinosine (ddI),2′,3′-dideoxy-3′-thiacytidine (3TC),2′,3′-didehydro-2′,3′-dideoxythymidine (d4T) and the monophosphatenucleotides of 3′-azido-3′-deoxythymidine (AZT),2′,3′-dideoxy-3′-thiacytidine (3TC) and2′,3′-didehydro-2′,3′-dideoxythymidine (d4T). In one embodiment,deoxynucleotides are used as the modifiers. When nucleotide modifiersare utilized, 1-3 nucleotide modifiers, or 2 nucleotide modifiers aresubstituted for the ribonucleotides on the 3′ end of the antisensestrand. When sterically hindered molecules are utilized, they areattached to the ribonucleotide at the 3′ end of the antisense strand.Thus, the length of the strand does not change with the incorporation ofthe modifiers. In another embodiment, the invention contemplatessubstituting two DNA bases in the dsRNA to direct the orientation ofDicer processing. In a further invention, two terminal DNA bases arelocated on the 3′ end of the antisense strand in place of tworibonucleotides forming a blunt end of the duplex on the 5′ end of thesense strand and the 3′ end of the antisense strand, and atwo-nucleotide RNA overhang is located on the 3′-end of the sensestrand. This is an asymmetric composition with DNA on the blunt end andRNA bases on the overhanging end.

The sense and antisense strands anneal under biological conditions, suchas the conditions found in the cytoplasm of a cell. In addition, aregion of one of the sequences, particularly of the antisense strand, ofthe dsRNA has a sequence length of at least 19 nucleotides, whereinthese nucleotides are adjacent to the 3′ end of antisense strand and aresufficiently complementary to a nucleotide sequence of the RNA producedfrom the target gene. Further in accordance with this embodiment, thedsRNA, i.e., the precursor RNAi molecule, may also have one or more ofthe following additional properties: (a) the antisense strand has aright shift from the typical 21 mer and (b) the strands may not becompletely complementary, i.e., the strands may contain simple mismatchpairings. A “typical” 21 mer siRNA is designed using conventionaltechniques, such as described above. This 21 mer is then used to designa right shift to include 1-7 additional nucleotides on the 5′ end of the21 mer. The sequence of these additional nucleotides may have anysequence. Although the added ribonucleotides may be complementary to thetarget gene sequence, full complementarity between the target sequenceand the antisense siRNA is not required. That is, the resultantantisense siRNA is sufficiently complementary with the target sequence.The first and second oligonucleotides are not required to be completelycomplementary. They only need to be substantially complementary toanneal under biological conditions and to provide a substrate for Dicerthat produces an siRNA sufficiently complementary to the targetsequence.

In one embodiment, the dsRNA has an asymmetric structure, with theantisense strand having a 25-base pair length, and the sense strandhaving a 27-base pair length with a 2 base 3′-overhang. In anotherembodiment, this dsRNA having an asymmetric structure further contains 2deoxynucleotides at the 3′ end of the antisense strand.

Suitable dsRNA compositions that contain two separate oligonucleotidescan be linked by a third structure. The third structure will not blockDicer activity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene. In oneembodiment, the third structure may be a chemical linking group. Manysuitable chemical linking groups are known in the art and can be used.Alternatively, the third structure may be an oligonucleotide that linksthe two oligonucleotides of the dsRNA is a manner such that a hairpinstructure is produced upon annealing of the two oligonucleotides makingup the dsRNA composition. The hairpin structure will not block Diceractivity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene.

One feature of the dsRNA compositions of the present invention is thatthey can serve as a substrate for Dicer. Typically, the dsRNAcompositions of this invention will not have been treated with Dicer,other RNases, or extracts that contain them. In the current inventionthis type of pretreatment can prevent Dicer annealing. Several methodsare known and can be used for determining whether a dsRNA compositionserves as a substrate for Dicer. For example, Dicer activity can bemeasured in vitro using the Recombinant Dicer Enzyme Kit (GTS, SanDiego, Calif.) according to the manufacturer's instructions. Diceractivity can be measured in vivo by treating cells with dsRNA andmaintaining them for 24 h before harvesting them and isolating theirRNA. RNA can be isolated using standard methods, such as with theRNeasy™ Kit (Qiagen) according to the manufacturer's instructions. Theisolated RNA can be separated on a 10% PAGE gel which is used to preparea standard RNA blot that can be probed with a suitable labeleddeoxyoligonucleotide, such as an oligonucleotide labeled with theStarfire® Oligo Labeling System (Integrated DNA Technologies, Inc.,Coralville, Iowa).

The effect that a dsRNA has on a cell can depend upon the cell itself.In some circumstances a dsRNA could induce apoptosis or gene silencingin one cell type and not another. Thus, it is possible that a dsRNAcould be suitable for use in one cell and not another. To be considered“suitable” a dsRNA composition need not be suitable under all possiblecircumstances in which it might be used, rather it need only be suitableunder a particular set of circumstances.

Modifications can be included in the dsRNA, i.e., the precursor RNAimolecule, of the present invention so long as the modification does notprevent the dsRNA composition from serving as a substrate for Dicer. Inone embodiment, one or more modifications are made that enhance Dicerprocessing of the dsRNA. In a second embodiment, one or moremodifications are made that result in more effective RNAi generation. Ina third embodiment, one or more modifications are made that support agreater RNAi effect. In a fourth embodiment, one or more modificationsare made that result in greater potency per each dsRNA molecule to bedelivered to the cell. Modifications can be incorporated in the3′-terminal region, the 5′-terminal region, in both the 3′-terminal and5′-terminal region or in some instances in various positions within thesequence. With the restrictions noted above in mind any number andcombination of modifications can be incorporated into the dsRNA. Wheremultiple modifications are present, they may be the same or different.Modifications to bases, sugar moieties, the phosphate backbone, andtheir combinations are contemplated. Either 5′-terminus can bephosphorylated.

Examples of modifications contemplated for the phosphate backboneinclude phosphonates, including methylphosphonate, phosphorothioate, andphosphotriester modifications such as alkylphosphotriesters, and thelike. Examples of modifications contemplated for the sugar moietyinclude 2′-alkyl pyrimidine, such as 2′-O-methyl, 2′-fluoro, amino, anddeoxy modifications and the like (see, e.g., Amarzguioui et al., 2003).Examples of modifications contemplated for the base groups includeabasic sugars, 2-O-alkyl modified pyrimidines, 4-thiouracil,5-bromouracil, 5-iodouracil, and 5-(3-aminoallyl)-uracil and the like.Locked nucleic acids, or LNA's, could also be incorporated. Many othermodifications are known and can be used so long as the above criteriaare satisfied. Examples of modifications are also disclosed in U.S. Pat.Nos. 5,684,143, 5,858,988 and 6,291,438 and in U.S. published patentapplication No. 2004/0203145 A1, each incorporated herein by reference.Other modifications are disclosed in Herdewijn (2000), Eckstein (2000),Rusckowski et al. (2000), Stein et al. (2001) and Vorobjev et al.(2001).

Additionally, the dsRNA structure can be optimized to ensure that theoligonucleotide segment generated from Dicer's cleavage will be theportion of the oligonucleotide that is most effective in inhibiting geneexpression. For example, in one embodiment of the invention a 27-bpoligonucleotide of the dsRNA structure is synthesized wherein theanticipated 21 to 22-bp segment that will inhibit gene expression islocated on the 3′-end of the antisense strand. The remaining baseslocated on the 5′-end of the antisense strand will be cleaved by Dicerand will be discarded. This cleaved portion can be homologous (i.e.,based on the sequence of the target sequence) or non-homologous andadded to extend the nucleic acid strand.

RNA may be produced enzymatically or by partial/total organic synthesis,and modified ribonucleotides can be introduced by in vitro enzymatic ororganic synthesis. In one embodiment, each strand is preparedchemically. Methods of synthesizing RNA molecules are known in the art,in particular, the chemical synthesis methods as described in Verma andEckstein (1998) or as described herein.

As is known, RNAi methods are applicable to a wide variety of genes in awide variety of organisms and the disclosed compositions and methods canbe utilized in each of these contexts. Examples of genes which can betargeted by the disclosed compositions and methods include endogenousgenes which are genes that are native to the cell or to genes that arenot normally native to the cell. Without limitation these genes includeoncogenes, cytokine genes, idiotype (Id) protein genes, prion genes,genes that expresses molecules that induce angiogenesis, genes foradhesion molecules, cell surface receptors, proteins involved inmetastasis, proteases, apoptosis genes, cell cycle control genes, genesthat express EGF and the EGF receptor, multi-drug resistance genes, suchas the MDR1 gene.

More specifically, the target mRNA of the invention specifies the aminoacid sequence of a cellular protein (e.g., a nuclear, cytoplasmic,transmembrane, or membrane-associated protein). In another embodiment,the target mRNA of the invention specifies the amino acid sequence of anextracellular protein (e.g., an extracellular matrix protein or secretedprotein). As used herein, the phrase “specifies the amino acid sequence”of a protein means that the mRNA sequence is translated into the aminoacid sequence according to the rules of the genetic code. The followingclasses of proteins are listed for illustrative purposes: developmentalproteins (e.g., adhesion molecules, cyclin kinase inhibitors, Wnt familymembers, Pax family members, Winged helix family members, Hox familymembers, cytokines/lymphokines and their receptors,growth/differentiation factors and their receptors, neurotransmittersand their receptors); oncogene-encoded proteins (e.g., ABLI, BCLI, BCL2,BCL6, CBFA2, CBL, CSFIR, ERBA, ERBB, EBRB2, ETSI, ETSI, ETV6, FGR, FOS,FYN, HCR, HRAS, JUN, KRAS, LCK, LYN, MDM2, MLL, MYB, MYC, MYCLI, MYCN,NRAS, PIM I, PML, RET, SRC, TALI, TCL3, and YES); tumor suppressorproteins (e.g., APC, BRCA1, BRCA2, MADH4, MCC, NF I, NF2, RB I, TP53,and WTI); and enzymes (e.g., ACC synthases and oxidases, ACP desaturasesand hydroxylases, ADP-glucose pyrophorylases, ATPases, alcoholdehydrogenases, amylases, amyloglucosidases, catalases, cellulases,chalcone synthases, chitinases, cyclooxygenases, decarboxylases,dextrinases, DNA and RNA polymerases, galactosidases, glucanases,glucose oxidases, granule-bound starch synthases, GTPases, helicases,hernicellulases, integrases, inulinases, invertases, isomerases,kinases, lactases, lipases, lipoxygenases, lysozymes, nopalinesynthases, octopine synthases, pectinesterases, peroxidases,phosphatases, phospholipases, phosphorylases, phytases, plant growthregulator synthases, polygalacturonases, proteinases and peptidases,pullanases, recombinases, reverse transcriptases, RUBISCOs,topoisomerases, and xylanases).

In one aspect, the target mRNA molecule of the invention specifies theamino acid sequence of a protein associated with a pathologicalcondition. For example, the protein may be a pathogen-associated protein(e.g., a viral protein involved in immunosuppression of the host,replication of the pathogen, transmission of the pathogen, ormaintenance of the infection), or a host protein which facilitates entryof the pathogen into the host, drug metabolism by the pathogen or host,replication or integration of the pathogen's genome, establishment orspread of infection in the host, or assembly of the next generation ofpathogen. Pathogens include RNA viruses such as flaviviruses,picornaviruses, rhabdoviruses, filoviruses, retroviruses, includinglentiviruses, or DNA viruses such as adenoviruses, poxviruses, herpesviruses, cytomegaloviruses, hepadnaviruses or others. Additionalpathogens include bacteria, fungi, helminths, schistosomes andtrypanosomes. Other kinds of pathogens can include mammaliantransposable elements. Alternatively, the protein may be atumor-associated protein or an autoimmune disease-associated protein.

The target gene may be derived from or contained in any organism. Theorganism may be a plant, animal, protozoa, bacterium, virus or fungus.See e.g., U.S. Pat. No. 6,506,559, incorporated herein by reference.

In another aspect, the present invention provides for a pharmaceuticalcomposition comprising the dsRNA of the present invention. The dsRNAsample can be suitably formulated and introduced into the environment ofthe cell by any means that allows for a sufficient portion of the sampleto enter the cell to induce gene silencing, if it is to occur. Manyformulations for dsRNA are known in the art and can be used so long asdsRNA gains entry to the target cells so that it can act. See, e.g.,U.S. published patent application Nos. 2004/0203145 A1 and 2005/0054598A1, each incorporated herein by reference. For example, dsRNA can beformulated in buffer solutions such as phosphate buffered salinesolutions, liposomes, micellar structures, and capsids. Formulations ofdsRNA with cationic lipids can be used to facilitate transfection of thedsRNA into cells. For example, cationic lipids, such as lipofectin (U.S.Pat. No. 5,705,188, incorporated herein by reference), cationic glycerolderivatives, and polycationic molecules, such as polylysine (publishedPCT International Application WO 97/30731, incorporated herein byreference), can be used. Suitable lipids include Oligofectamine,Lipofectamine (Life Technologies), NC388 (Ribozyme Pharmaceuticals,Inc., Boulder, Colo.), or FuGene 6 (Roche) all of which can be usedaccording to the manufacturer's instructions.

It can be appreciated that the method of introducing dsRNA into theenvironment of the cell will depend on the type of cell and the make upof its environment. For example, when the cells are found within aliquid, one preferable formulation is with a lipid formulation such asin lipofectamine and the dsRNA can be added directly to the liquidenvironment of the cells. Lipid formulations can also be administered toanimals such as by intravenous, intramuscular, or intraperitonealinjection, or orally or by inhalation or other methods as are known inthe art. When the formulation is suitable for administration intoanimals such as mammals and more specifically humans, the formulation isalso pharmaceutically acceptable. Pharmaceutically acceptableformulations for administering oligonucleotides are known and can beused. In some instances, it may be preferable to formulate dsRNA in abuffer or saline solution and directly inject the formulated dsRNA intocells, as in studies with oocytes. The direct injection of dsRNAduplexes may also be done. For suitable methods of introducing dsRNA seeU.S. published patent application No. 2004/0203145 A1, incorporatedherein by reference.

Suitable amounts of dsRNA must be introduced and these amounts can beempirically determined using standard methods. Typically, effectiveconcentrations of individual dsRNA species in the environment of a cellwill be about 50 nanomolar or less 10 nanomolar or less, or compositionsin which concentrations of about 1 nanomolar or less can be used. Inother embodiment, methods utilize a concentration of about 200 picomolaror less and even a concentration of about 50 picomolar or less can beused in many circumstances.

The method can be carried out by addition of the dsRNA compositions toany extracellular matrix in which cells can live provided that the dsRNAcomposition is formulated so that a sufficient amount of the dsRNA canenter the cell to exert its effect. For example, the method is amenablefor use with cells present in a liquid such as a liquid culture or cellgrowth media, in tissue explants, or in whole organisms, includinganimals, such as mammals and especially humans.

Expression of a target gene can be determined by any suitable method nowknown in the art or that is later developed. It can be appreciated thatthe method used to measure the expression of a target gene will dependupon the nature of the target gene. For example, when the target geneencodes a protein the term “expression” can refer to a protein ortranscript derived from the gene. In such instances the expression of atarget gene can be determined by measuring the amount of mRNAcorresponding to the target gene or by measuring the amount of thatprotein. Protein can be measured in protein assays such as by stainingor immunoblotting or, if the protein catalyzes a reaction that can bemeasured, by measuring reaction rates. All such methods are known in theart and can be used. Where the gene product is an RNA species expressioncan be measured by determining the amount of RNA corresponding to thegene product. Several specific methods for detecting gene expression aredescribed in Example 1. The measurements can be made on cells, cellextracts, tissues, tissue extracts or any other suitable sourcematerial.

The determination of whether the expression of a target gene has beenreduced can be by any suitable method that can reliably detect changesin gene expression. Typically, the determination is made by introducinginto the environment of a cell undigested dsRNA such that at least aportion of that dsRNA enters the cytoplasm and then measuring theexpression of the target gene. The same measurement is made on identicaluntreated cells and the results obtained from each measurement arecompared. When the method appears to reduce the expression of the targetgene by about 10% or more (which is equivalent to about 90% or less) ofthe level in an untreated organism, for purposes of this invention, themethod is considered to reduce the expression of the target gene.Typically, the method can be used to reduce that expression of a targetgene by far more than 10%. In some instances the method can be used toreduce the expression by about 50% or more, in more preferred methodsthe expression is reduced by about 75% or more, still more preferableare methods that reduce the expression by about 90% or more, or evenabout 95% or more, or about 99% or more or even by completelyeliminating expression of the target gene.

The dsRNA can be formulated as a pharmaceutical composition whichcomprises a pharmacologically effective amount of a dsRNA andpharmaceutically acceptable carrier. A pharmacologically ortherapeutically effective amount refers to that amount of a dsRNAeffective to produce the intended pharmacological, therapeutic orpreventive result. The phrases “pharmacologically effective amount” and“therapeutically effective amount” or simply “effective amount” refer tothat amount of an RNA effective to produce the intended pharmacological,therapeutic or preventive result. For example, if a given clinicaltreatment is considered effective when there is at least a 20% reductionin a measurable parameter associated with a disease or disorder, atherapeutically effective amount of a drug for the treatment of thatdisease or disorder is the amount necessary to effect at least a 20%reduction in that parameter.

The phrase “pharmaceutically acceptable carrier” refers to a carrier forthe administration of a therapeutic agent. Exemplary carriers includesaline, buffered saline, dextrose, water, glycerol, ethanol, andcombinations thereof. For drugs administered orally, pharmaceuticallyacceptable carriers include, but are not limited to pharmaceuticallyacceptable excipients such as inert diluents, disintegrating agents,binding agents, lubricating agents, sweetening agents, flavoring agents,coloring agents and preservatives. Suitable inert diluents includesodium and calcium carbonate, sodium and calcium phosphate, and lactose,while corn starch and alginic acid are suitable disintegrating agents.Binding agents may include starch and gelatin, while the lubricatingagent, if present, will generally be magnesium stearate, stearic acid ortalc. If desired, the tablets may be coated with a material such asglyceryl monostearate or glyceryl distearate, to delay absorption in thegastrointestinal tract. The pharmaceutically acceptable carrier of thedisclosed dsRNA composition may be micellar structures, such as aliposomes, capsids, capsoids, polymeric nanocapsules, or polymericmicrocapsules.

Polymeric nanocapsules or microcapsules facilitate transport and releaseof the encapsulated or bound dsRNA into the cell. They include polymericand monomeric materials, especially including polybutylcyanoacrylate. Asummary of materials and fabrication methods has been published (seeKreuter, 1991). The polymeric materials which are formed from monomericand/or oligomeric precursors in the polymerization/nanoparticlegeneration step, are per se known from the prior art, as are themolecular weights and molecular weight distribution of the polymericmaterial which a person skilled in the field of manufacturingnanoparticles may suitably select in accordance with the usual skill.

Suitably formulated pharmaceutical compositions of this invention can beadministered by any means known in the art such as by parenteral routes,including intravenous, intramuscular, intraperitoneal, subcutaneous,transdermal, airway (aerosol), rectal, vaginal and topical (includingbuccal and sublingual) administration. In some embodiments, thepharmaceutical compositions are administered by intravenous orintraparenteral infusion or injection.

In general a suitable dosage unit of dsRNA will be in the range of 0.001to 0.25 milligrams per kilogram body weight of the recipient per day, orin the range of 0.01 to 20 micrograms per kilogram body weight per day,or in the range of 0.01 to 10 micrograms per kilogram body weight perday, or in the range of 0.10 to 5 micrograms per kilogram body weightper day, or in the range of 0.1 to 2.5 micrograms per kilogram bodyweight per day. Pharmaceutical composition comprising the dsRNA can beadministered once daily. However, the therapeutic agent may also bedosed in dosage units containing two, three, four, five, six or moresub-doses administered at appropriate intervals throughout the day. Inthat case, the dsRNA contained in each sub-dose must be correspondinglysmaller in order to achieve the total daily dosage unit. The dosage unitcan also be compounded for a single dose over several days, e.g., usinga conventional sustained release formulation which provides sustainedand consistent release of the dsRNA over a several day period. Sustainedrelease formulations are well known in the art. In this embodiment, thedosage unit contains a corresponding multiple of the daily dose.Regardless of the formulation, the pharmaceutical composition mustcontain dsRNA in a quantity sufficient to inhibit expression of thetarget gene in the animal or human being treated. The composition can becompounded in such a way that the sum of the multiple units of dsRNAtogether contain a sufficient dose.

Data can be obtained from cell culture assays and animal studies toformulate a suitable dosage range for humans. The dosage of compositionsof the invention lies within a range of circulating concentrations thatinclude the ED₅₀ (as determined by known methods) with little or notoxicity. The dosage may vary within this range depending upon thedosage form employed and the route of administration utilized. For anycompound used in the method of the invention, the therapeuticallyeffective dose can be estimated initially from cell culture assays. Adose may be formulated in animal models to achieve a circulating plasmaconcentration range of the compound that includes the IC₅₀ (i.e., theconcentration of the test compound which achieves a half-maximalinhibition of symptoms) as determined in cell culture. Such informationcan be used to more accurately determine useful doses in humans. Levelsof dsRNA in plasma may be measured by standard methods, for example, byhigh performance liquid chromatography.

In a further aspect, the present invention relates to a method fortreating a subject having a disease or at risk of developing a diseasecaused by the expression of a target gene. In this embodiment, the dsRNAcan act as novel therapeutic agents for controlling one or more ofcellular proliferative and/or differentiative disorders, disordersassociated with bone metabolism, immune disorders, hematopoieticdisorders, cardiovascular disorders, liver disorders, viral diseases, ormetabolic disorders. The method comprises administering a pharmaceuticalcomposition of the invention to the patient (e.g., human), such thatexpression of the target gene is silenced. Because of their highspecificity, the dsRNAs of the present invention specifically targetmRNAs of target genes of diseased cells and tissues.

In the prevention of disease, the target gene may be one which isrequired for initiation or maintenance of the disease, or which has beenidentified as being associated with a higher risk of contracting thedisease. In the treatment of disease, the dsRNA can be brought intocontact with the cells or tissue exhibiting the disease. For example,dsRNA substantially identical to all or part of a mutated geneassociated with cancer, or one expressed at high levels in tumor cells,e.g. aurora kinase, may be brought into contact with or introduced intoa cancerous cell or tumor gene.

Examples of cellular proliferative and/or differentiative disordersinclude cancer, e.g., carcinoma, sarcoma, metastatic disorders orhematopoietic neoplastic disorders, e.g., leukemias. A metastatic tumorcan arise from a multitude of primary tumor types, including but notlimited to those of prostate, colon, lung, breast and liver origin. Asused herein, the terms “cancer,” “hyperproliferative,” and “neoplastic”refer to cells having the capacity for autonomous growth, i.e., anabnormal state of condition characterized by rapidly proliferating cellgrowth. These terms are meant to include all types of cancerous growthsor oncogenic processes, metastatic tissues or malignantly transformedcells, tissues, or organs, irrespective of histopathologic type or stageof invasiveness. Proliferative disorders also include hematopoieticneoplastic disorders, including diseases involvinghyperplastic/neoplatic cells of hematopoietic origin, e.g., arising frommyeloid, lymphoid or erythroid lineages, or precursor cells thereof.

The present invention can also be used to treat a variety of immunedisorders, in particular those associated with overexpression of a geneor expression of a mutant gene. Examples of hematopoietic disorders ordiseases include, without limitation, autoimmune diseases (including,for example, diabetes mellitus, arthritis (including rheumatoidarthritis, juvenile rheumatoid arthritis, osteoarthritis, psoriaticarthritis), multiple sclerosis, encephalomyelitis, myasthenia gravis,systemic lupus erythematosis, autoimmune thyroiditis, dermatitis(including atopic dermatitis and eczematous dermatitis), psoriasis,Sjogren's Syndrome, Crohn's disease, aphthous ulcer, iritis,conjunctivitis, keratoconjunctivitis, ulcerative colitis, asthma,allergic asthma, cutaneous lupus erythematosus, scleroderma, vaginitis,proctitis, drug eruptions, leprosy reversal reactions, erythema nodosumleprosum, autoimmune uveitis, allergic encephalomyclitis, acutenecrotizing hemorrhagic encephalopathy, idiopathic bilateral progressivesensorineural hearing, loss, aplastic anemia, pure red cell anemia,idiopathic thrombocytopenia, polychondritis, Wegener's granulomatosis,chronic active hepatitis, Stevens-Johnson syndrome, idiopathic sprue,lichen planus, Graves' disease, sarcoidosis, primary biliary cirrhosis,uveitis posterior, and interstitial lung fibrosis), graft-versus-hostdisease, cases of transplantation, and allergy.

In another embodiment, the invention relates to a method for treatingviral diseases, including but not limited to human papilloma virus,hepatitis C, hepatitis B, herpes simplex virus (HSV), HIV-AIDS,poliovirus, and smallpox virus. dsRNAs of the invention are prepared asdescribed herein to target expressed sequences of a virus, thusameliorating viral activity and replication. The molecules can be usedin the treatment and/or diagnosis of viral infected tissue, both animaland plant. Also, such molecules can be used in the treatment ofvirus-associated carcinoma, such as hepatocellular cancer.

The dsRNA of the present invention can also be used to inhibit theexpression of the multi-drug resistance 1 gene (“MDR1”). “Multi-drugresistance” (MDR) broadly refers to a pattern of resistance to a varietyof chemotherapeutic drugs with unrelated chemical structures anddifferent mechanisms of action. Although the etiology of MDR ismultifactorial, the overexpression of P-glycoprotein (Pgp), a membraneprotein that mediates the transport of MDR drugs, remains the mostcommon alteration underlying MDR in laboratory models (Childs and Ling,1994). Moreover, expression of Pgp has been linked to the development ofMDR in human cancer, particularly in the leukemias, lymphomas, multiplemyeloma, neuroblastoma, and soft tissue sarcoma (Fan et al.). Recentstudies showed that tumor cells expressing MDR-associated protein (MRP)(Cole et al., 1992), lung resistance protein (LRP) (Scheffer et al.,1995) and mutation of DNA topoisomerase II (Beck, 1989) also may renderMDR.

The practice of the present invention employs, unless otherwiseindicated, conventional techniques of chemistry, molecular biology,microbiology, recombinant DNA, genetics, immunology, cell biology, cellculture and transgenic biology, which are within the skill of the art.See, e.g., Maniatis et al., 1982, Molecular Cloning (Cold Spring HarborLaboratory Press, Cold Spring Harbor, N.Y.); Sambrook et al., 1989,Molecular Cloning, 2nd Ed. (Cold Spring Harbor Laboratory Press, ColdSpring Harbor, N.Y.); Sambrook and Russell, 2001, Molecular Cloning, 3rdEd. (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.);Ausubel et al., 1992), Current Protocols in Molecular Biology (JohnWiley & Sons, including periodic updates); Glover, 1985, DNA Cloning(IRL Press, Oxford); Anand, 1992; Guthrie and Fink, 1991; Harlow andLane, 1988, Antibodies, (Cold Spring Harbor Laboratory Press, ColdSpring Harbor, N.Y.); Jakoby and Pastan, 1979; Nucleic AcidHybridization (B. D. Hames & S. J. Higgins eds. 1984); Transcription AndTranslation (B. D. Hames & S. J. Higgins eds. 1984); Culture Of AnimalCells (R. I. Freshney, Alan R. Liss, Inc., 1987); Immobilized Cells AndEnzymes (IRL Press, 1986); B. Perbal, A Practical Guide To MolecularCloning (1984); the treatise, Methods In Enzymology (Academic Press,Inc., N.Y.); Gene Transfer Vectors For Mammalian Cells (J. H. Miller andM. P. Calos eds., 1987, Cold Spring Harbor Laboratory); Methods InEnzymology, Vols. 154 and 155 (Wu et al. eds.), Immunochemical MethodsIn Cell And Molecular Biology (Mayer and Walker, eds., Academic Press,London, 1987); Handbook Of Experimental Immunology, Volumes I-IV (D. M.Weir and C. C. Blackwell, eds., 1986); Riott, Essential Immunology, 6thEdition, Blackwell Scientific Publications, Oxford, 1988; Hogan et al.,Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press,Cold Spring Harbor, N.Y., 1986); Westerfield, M., The zebrafish book. Aguide for the laboratory use of zebrafish (Danio rerio), (4th Ed., Univ.of Oregon Press, Eugene, 2000).

EXAMPLES

The present invention is described by reference to the followingExamples, which are offered by way of illustration and are not intendedto limit the invention in any manner. Standard techniques well known inthe art or the techniques specifically described below were utilized.

Example 1 Preparation of Double-Stranded RNA Oligonucleotides

Oligonucleotide Synthesis and Purification

RNA oligonucleotides were synthesized using solid phase phosphoramiditechemistry, deprotected and desalted on NAP-5 columns (Amersham PharmaciaBiotech, Piscataway, N.J.) using standard techniques (Damha and Olgivie,1993; Wincott et al., 1995). The oligomers were purified usingion-exchange high performance liquid chromatography (IE-HPLC) on anAmersham Source 15Q column (1.0 cm×25 cm) (Amersham Pharmacia Biotech,Piscataway, N.J.) using a 15 min step-linear gradient. The gradientvaried from 90:10 Buffers A:B to 52:48 Buffers A:B, where Buffer A was100 mM Tris pH 8.5 and Buffer B was 100 mM Tris pH 8.5, 1 M NaCl.Samples were monitored at 260 nm and peaks corresponding to thefull-length oligonucleotide species were collected, pooled, desalted onNAP-5 columns, and lyophilized.

The purity of each oligomer was determined by capillary electrophoresis(CE) on a Beckman PACE 5000 (Beckman Coulter, Inc., Fullerton, Calif.).The CE capillaries had a 100 μm inner diameter and contained ssDNA 100RGel (Beckman-Coulter). Typically, about 0.6 nmole of oligonucleotide wasinjected into a capillary, ran in an electric field of 444 V/cm anddetected by UV absorbance at 260 nm. Denaturing Tris-Borate-7 M-urearunning buffer was purchased from Beckman-Coulter. Oligoribonucleotideswere at least 90% pure as assessed by CE for use in experimentsdescribed below. Compound identity was verified by matrix-assisted laserdesorption ionization time-of-flight (MALDI-TOF) mass spectroscopy on aVoyager DE™ Biospectometry Work Station (Applied Biosystems, FosterCity, Calif.) following the manufacturer's recommended protocol.Relative molecular masses of all oligomers were within 0.2% of expectedmolecular mass.

Preparation of Duplexes

Single-stranded RNA (ssRNA) oligomers were resuspended at 100 μMconcentration in duplex buffer consisting of 100 mM potassium acetate,30 mM HEPES, pH 7.5. Complementary sense and antisense strands weremixed in equal molar amounts to yield a final solution of 50 μM duplex.Samples were heated to 95° C. for 5′ and allowed to cool to roomtemperature before use. Double-stranded RNA (dsRNA) oligomers werestored at −20° C. Single-stranded RNA oligomers were stored lyophilizedor in nuclease-free water at −80° C.

Nomenclature

For consistency, the following nomenclature has been employed throughoutthe specification and Examples. Names given to duplexes indicate thelength of the oligomers and the presence or absence of overhangs. A“21+2” duplex contains two RNA strands both of which are 21 nucleotidesin length, also termed a 21 mer siRNA duplex, and having a 2 base3′-overhang. A “21-2” design is a 21 mer siRNA duplex with a 2 base5′-overhang. A 21-0 design is a 21 mer siRNA duplex with no overhangs(blunt). A “21+2UU” is a 21 mer duplex with 2-base 3′-overhang and theterminal 2 bases at the 3′-ends are both U residues (which may result inmismatch with target sequence). A “ 25/27” is an asymmetric duplexhaving a 25 base sense strand and a 27 base antisense strand with a2-base 3′-overhang. A “ 27/25” is an asymmetric duplex having a 27 basesense strand and a 25 base antisense strand.

Example 2 Increased Potency of 25 Mers

This example demonstrates that dsRNAs having strands that are 25nucleotides in length or longer have surprisingly increased potency inmammalian systems than known 21 mer to 23 mer siRNAs.

During investigations of the effects of different 5′ and 3′ endstructures of dsRNAs made through bacteriophage T7 in vitrotranscription (Kim et al., 2004), we observed that some seemed to havegreater potency than synthetic 21 mer siRNAs directed to the same targetsite, and that this property seemed to correlate with length. To furtherexplore this phenomenon, we systematically studied the silencingproperties of chemically synthesized duplex RNAs of different lengthsand designs.

Cell Culture, Transfection, and EGFP Assays

HEK 293 cells were split in 24-well plates to 60% confluency in DMEMmedium 1 d before transfection. After adding the aliquot of each RNA, 50μl of Opti Media containing the reporter vectors was added. Next, 50 μlof Opti Media containing 1.5 μl of Lipofectamine 2000 (Invitrogen) wasmixed and incubated for 15 min. The cells were then added in 0.4 ml ofDMEM medium. To normalize for transfection efficiency, each assayincluded cotransfection of the target and/or duplex RNAs with eitherfirefly luciferase or a red fluorescent protein (RFP) reporter plasmid(all other assays). For the luciferase assay, the Steady Glo Luciferaseassay kit was used according to manufacturer's instructions (Promega).For RFP cotransfection, the indicated amount of EGFP reporter plasmid(pLEGFP-C1 vector, Clontech) was transfected with 20 ng of RFP reporterplasmid (pDsRed2-C1, BD Sciences). After 24 h, RFP expression wasmonitored by fluorescence microscopy. Only experiments wheretransfection efficiency was >90% (as assessed by RFP expression) wereevaluated. EGFP expression was measured 24 h later. EGFP expression wasdetermined either from the median number of EGFP-fluorescent cellsdetermined by FACS (live cells) or by fluorometer readings (cellextracts).

For EGFP assays using NIH3T3 cells stably expressing EGFP, measurementswere determined using a VersaFluor Fluorometer (Bio-Rad) usingexcitation filter D490 and emission filter D520. Before transfections,cells were seeded to approximately 30% confluency in a 24-well plate. Onday 0, cells were transfected as described above and the medium waschanged on day 1 after transfection. The first EGFP assay was carriedout on day 3. For extract preparation 1×10⁵ cells were taken and 1×10⁴cells were further propagated for the day 6 EGFP assays. On days 6 and 9the same procedure was repeated.

Extract measurements of EGFP were obtained as follows: 1×10⁵ cells weresuspended in 300 μl of PBS and sonicated for 10 s followed by a 2-minmicrocentrifugation. The supernatants were used for fluorescencemeasurements. Percentages of EGFP expression were determined relative toextracts prepared from untreated NIH3T3 cells.

Nucleic Acid Reagents

The reporter system employed EGFP either as a transfection plasmidvector pEGFP-Cl (Clontech, Palo Alto, Calif.) or as a stabletransformant in an NIH 3T3 cell line. The coding sequence of EGFP isshown in Table 1, from Genbank accession #U55763. The ATG start codonand TAA stop codons are highlighted in bold font and sites target bysiRNA reagents in this Example and other Examples are underscored.

TABLE 1 Nucleotide Sequence of EGFP (SEQ ID NO: 1)atggtgagcaagggcgaggagctgttcaccggggtggtgoccatcctggtcgagctggacggcgacgtaaacggccacaagttcagcgtgtccggcgagggcgagggcgatgccacctacggcaagctgaccctgaagttcatctgcaccaccggcaagctgcccgtgccctggcccaccctcgtgaccaccctgacctacggcgtgcagtgcttcagccgctaccccgaccacatgaagcagcacgacttcttcaagtccgccatgcccgaaggctacgtccaggagcgcaccatcttcttcaaggacgacggcaactacaagacccgcgccgaggtgaagttcgagggcgacaccctggtgaaccgcatcgagctgaagggcatcgacttcaaggaggacggcaacatcctggggcacaagctggagtacaactacaacagccacaacgtctatatcatggccgacaagcagaagaacggcatcaaggtgaacttcaagatccgccacaacatcgaggacggcagcgtgcagctcgccgaccactaccagcagaacacccccatcggcgacggccccgtgctgctgcccgacaaccactacctgagcacccagtccgccctgagcaaagaccccaacgagaagcgcgatcacatggtcctgctggagttcgtgaccgccgccgggatcactctcggca tggacgagctgtacaagtaa

Site-1 used for siRNA targeting in EGFP for this example was:

SITE 1: (SEQ ID NO: 2) 5′ GCAAGCUGACCCUGAAGUUCAUCUGCACCACCGGCAAGC 3′.

RNA duplexes were synthesized and prepared as described in Example 1.RNA duplexes targeting EGFP Site-1 are summarized in Table 2. Somesequences had the dinucleotide sequence “UU” placed at the 3′-end of thesense strand (Elbashir et al., 2001c; Hohjoh, 2002). Mismatches thatresulted from including 3′-terminal “UU” or where a mismatch wasintentionally positioned are highlighted in bold and underscored.

TABLE 2 Summary of Oligonucleotide Reagents, EGFP Site-1 Sequence NameSEQ ID NO. 5′ GCAAGCUGACCCUGAAGUUCAUCUGCACCACCGGCAAGC 3′ EGFP Site-1SEQ ID NO: 2 5′ GCAAGCUGACCCUGAAGUUCA EGFPS1-21-2 SEQ ID No: 3 3′  UUCGACUGGGACUUCAAGUAG SEQ ID NO: 4 5′   AAGCUGACCCUGAAGUUCAUCEGFPS1-21 + 0 SEQ ID No: 5 3′   UUCGACUGGGACUUCAAGUAG SEQ ID No: 6 5′          CCUGAAGUUCAUCUGCACCAC EGFPS1-21 + 2(1) SEQ ID No: 7 3′        UGGGACUUCAAGUAGACGUGG SEQ ID No: 8 5′         CCCUGAAGUUCAUCUGCACCA EGFPS1-21 + 2(2) SEQ ID No: 9 3′       CUGGGACUUCAAGUAGACGUG SEQ ID No: 10 5′        ACCCUGAAGUUCAUCUGCACC EGFPS1-21 + 2(3) SEQ ID No: 11 3′      ACUGGGACUUCAAGUAGACGU SEQ ID No: 12 5′       GACCCUGAAGUUCAUCUGCAC EGFPS1-21 + 2(4) SEQ ID No: 13 3′     GACUGGGACUUCAAGUAGACG SEQ ID No: 14 5′       UGACCCUGAAGUUCAUCUGCAEGFPS1-21 + 2(5) SEQ ID No: 15 3′     CGACUGGGACUUCAAGUAGACSEQ ID No: 16 5′      CUGACCCUGAAGUUCAUCUGC EGFPS1-21 + 2(6)SEQ ID No: 17 3′    UCGACUGGGACUUCAAGUAGA SEQ ID No: 18 5′    GCUGACCCUGAAGUUCAUCUG EGFPS1-21 + 2(7) SEQ ID No: 19 3′  UUCGACUGGGACUUCAAGUAG SEQ ID No: 20 5′ GCAAGCUGACCCUGAAGUUCAU UEGFPS1-23 − 2 UU SEQ ID No: 21 3′   UUCGACUGGGACUUCAAGUAGACSEQ ID No: 22 5′     GCUGACCCUGAAGUUCAUCUGUU EGFPS1-23 + 2 UUSEQ ID No: 23 3′   UUCGACUGGGACUUCAAGUAGAC SEQ ID No: 24 5′GCAAGCUGACCCUGAAGUUCAU U U EGFPS1-24 − 2 UU SEQ ID No: 25 3′  UUCGACUGGGACUUCAAGUAGACG SEQ ID No: 26 5′     GCUGACCCUGAAGUUCAUCUGCUUEGFPS1-24 + 2 UU SEQ ID No: 27 3′   UUCGACUGGGACUUCAAGUAGACGSEQ ID No: 28 5′ GCAAGCUGACCCUGAAGUUCAUCU U EGFPS1-25 − 2 UUSEQ ID No: 29 3′   UUCGACUGGGACUUCAAGUAGACGU SEQ ID No: 30 5′    GCUGACCCUGAAGUUCAUCUGCAUU EGFPS1-25 + 2 UU SEQ ID No: 31 3′  UUCGACUGGGACUUCAAGUAGACGU SEQ ID No: 32 5′  AAGCUGACCCUGAAGUUCAUCUGCAC EGFPS1-26 + 0 SEQ ID No: 33 3′  UUCGACUGGGACUUCAAGUAGACGUG SEQ ID No: 34 5′   AAGCUGACCCUGAAGUUCAUCUGCUU EGFPS1-26 + 0 UU SEQ ID No: 35 3′   UUCGACUGGGACUUCAAGUAGACGUGSEQ ID No: 36 5′ GCAAGCUGACCCUGAAGUUCAUCU UU EGFPS1-26 − 2 UUSEQ ID No: 37 3′   UUCGACUGGGACUUCAAGUAGACGUG SEQ ID No: 38 5′    GCUGACCCUGAAGUUCAUCUGCACUU EGFPS1-26 + 2 UU SEQ ID No: 39 3′  UUCGACUGGGACUUCAAGUAGACGUG SEQ ID No: 40 5′  AAGCUGACCCUGAAGUUCAUCUGCACC EGFPS1-27 + 0 SEQ ID No: 41 3′  UUCGACUGGGACUUCAAGUAGACGUGG SEQ ID No: 42 5′F-AAGCUGACCCUGAAGUUCAUCUGCACC EGFPS1-27 + 0 SEQ ID No: 43 3′  UUCGACUGGGACUUCAAGUAGACGUGG-F FAM #1 SEQ ID No: 44 5′  AAGCUGACCCUGAAGUUCAUCUGCACC-F EGFPS1-27 + 0 SEQ ID No: 45 3′  UUCGACUGGGACUUCAAGUAGACGUGG-F FAM #2 SEQ ID No: 44 5′F-AAGCUGACCCUGAAGUUCAUCUGCACC EGFPS1-27 + 0 SEQ ID No: 43 3′F-UUCGACUGGGACUUCAAGUAGACGUGG FAM #4 SEQ ID No: 46 5′  AAGCUGACCCUGAAGUUCAUCUGCACC-F EGFPS1-27 + 0 SEQ ID No: 45 3′F-UUCGACUGGGACUUCAAGUAGACGUGG FAM #5 SEQ ID No: 46 5′  AAGCUGACCCUGAAGUUCAUCUGCA UU EGFPS1-27 + 0 UU SEQ ID No: 47 3′  UUCGACUGGGACUUCAAGUAGACGUGG SEQ ID No: 48 5′ GCAAGCUGACCCUGAAGUUCAUCUGUU EGFPS1-27 − 2 UU SEQ ID No: 49 3′   UUCGACUGGGACUUCAAGUAGACGUGGSEQ ID No: 50 5′     GCUGACCCUGAAGUUCAUCUGCAC A UU EGFPS1-27 + 2 UU/SEQ ID No: 51 3′   UUCGACUGGGACUUCAAGUAGACGUGG 25 SEQ ID No: 52 5′  AAGCUGACCCUGAAG A UCAUCUGCA UU EGFPS1-27 + 0 UU/ SEQ ID No: 53 3′  UUCGACUGGGACUUC U AGUAGACGUGG 16 SEQ ID No: 54 5′   AAGCUGACCCUGAAG AACAUCUGCA UU EGFPS1-27 + 0 UU/ SEQ ID No: 55 3′   UUCGACUGGGACUUC UUGUAGACGUGG 16, 17 SEQ ID No: 56 5′   AAGCUGACCCUGAA C AA CAUCUGCA UUEGFPS1-27 + 0 UU/ SEQ ID No: 57 3′   UUCGACUGGGACUU GUU GUAGACGUGG15, 16, 17 SEQ ID No: 58 5′   AAGCUGACCCUG UUCA UCAUCUGCACC EGFPS1-27 +0-mut SEQ ID No: 59 3′   UUCGACUGGGAC AAGU AGUAGACGUGG SEQ ID No: 60 5′  AAGCUGACCCUGAAGUUCAUCUGCACCA EGFPS1-28 + 0 SEQ ID No: 61 3′  UUCGACUGGGACUUCAAGUAGACGUGGU SEQ ID No: 62 5′  AAGCUGACCCUGAAGUUCAUCUGCACCAC EGFPS1-29 + 0 SEQ ID No: 63 3′  UUCGACUGGGACUUCAAGUAGACGUGGUG SEQ ID No: 64 5′  AAGCUGACCCUGAAGUUCAUCUGCACCACC EGFPS1-30 + 0 SEQ ID No: 65 3′  UUCGACUGGGACUUCAAGUAGACGUGGUGG SEQ ID No: 66 5′  AAGCUGACCCUGAAGUUCAUCUGCACCACCGGCAA EGFPS1-35 + 0 SEQ ID NO: 67 3′  UUCGACUGGGACUUCAAGUAGACGUGGUGGCCGUU SEQ ID NO: 68 5′  AAGCUGACCCUGAAGUUCAUCUGCACCACCGGCAAGCUGC EGFPS1-40 + 0 SEQ ID NO: 693′   UUCGACUGGGACUUCAAGUAGACGUGGUGGCCGUUCGACG SEQ ID NO: 70 5′  AAGCUGACCCUGAAGUUCAUCUGCACCACCGGCAAGCUGCCCGUG EGFPS1-45 + 0SEQ ID NO: 71 3′   UUCGACUGGGACUUCAAGUAGACGUGGUGGCCGUUCGACGGGCACSEQ ID NO: 72

Results

An expanded set of synthetic RNA duplexes of varying length containing3′ overhangs, 5′ overhangs or blunt ends were tested for their relativepotency in a model system targeting ‘site 1’ in EGFP (Kim et al., 2003).We carried out our initial transfections using 50 nM of the various RNAduplexes (FIG. 1A). The real potency of the longer duplexes was shownonly when transfections were done at subnanomolar concentrations. Usingduplex RNA concentrations of 200 μM (FIG. 1B) and 50 μM (FIG. 1C), weobserved that potency increased with length. Blunt, 5′-overhanging and3′-overhanging ends were of similar potency. Increased potencies of thelonger duplex RNAs were observed even in the NIH3T3 cells stablyexpressing EGFP (FIG. 1F). Duplexes longer than 27 nt were synthesizedand tested for RNAi efficacy (FIG. 1D). Maximal inhibitory activity wasseen at a duplex length of 27 bp (FIG. 1D). Longer duplexes showedprogressive loss of functional RNAi activity and by 40-45 bp were whollyinactive at the tested concentrations, which also correlated with poorin vitro cleavage of these duplexes by Dicer (FIG. 1E).

Example 3 Dicer Substrates

This example demonstrates a method for determining whether a dsRNAserves as a substrate for Dicer.

In vitro Dicer Cleavage Assays

RNA duplexes (100 pmol) were incubated in 20 μl of 20 mM Tris pH 8.0,200 mM NaCl, 2.5 mM MgCl₂ with 1 unit of recombinant human Dicer(Stratagene) for 24 h. A 3 μl aliquot of each reaction (15 pmol RNA) wasseparated in a 15% nondenaturing polyacrylamide gel, stained withGelStar (Ambrex) and visualized using UV excitation.

Results

Incubation of 21-bp to 27-bp RNA duplexes with recombinant human Dicerresulted in cleavage of the 23 mer, 25 mer and 27 mer duplexes, but notthe 21 mer duplex (FIG. 2A). Determinations of relative efficiencies ofDicer cleavage were not possible under the in vitro conditionsrecommended by the supplier owing to the slow kinetics of this reaction.Aside from the possibility that the dsRNAs longer than 30 bp may need tobe preprocessed by Drosha, a micro RNA processing enzyme, to be goodsubstrates for Dicer, we do not have an explanation for why these longerdsRNAs are both poor substrates for Dicer and poor triggers for RNAi.

Example 4 Effect of End-Modification on Dicer Activity

The effect of end-modification of dsRNA was tested using the in vitroDicer cleavage assay described in Example 3.

For the 27 mer duplexes, we tested the effects of fluoresceinend-modification on Dicer activity and the ability to trigger RNAisilencing. RNA oligonucleotides were synthesized for the EGFPS1 sitewith 6-carboxyfluorescein (6FAM) attached to the 5′ end of the sense (S)strand, 3′ end of the S-strand, 5′ end of the antisense (AS) strand and3′ end of the AS strand. Pairwise combinations were used to make duplexRNAs (FIG. 2B). Duplex 3 was the unmodified wild-type EGFPS1 27+0duplex. The structure of the 6FAM modified duplexes are shown in Table2. RNA duplexes were incubated for 24 h with recombinant human Dicer,separated by nondenaturing polyacrylamide gel electrophoresis (PAGE),stained and visualized by UV excitation (FIG. 2C). Unlike in earlierexperiments, in which RNA duplexes were fully cleaved during a 24-hincubation (FIG. 2A), all of the modified duplexes showed some degree ofresistance to cleavage by Dicer. Only the unmodified wild-type sequence(duplex 3) was fully cleaved in the in vitro Dicer reaction. The duplexbearing a 3′-6FAM on the S strand and a 3′-6FAM on the AS strand (duplex5) was totally resistant to cleavage under these conditions. Functionalpotencies of these five duplexes were compared in EGFP cotransfectionassays (FIG. 2D) using 200 pM RNA concentrations. Parallel to thepatterns seen for in vitro Dicer cleavage, all of the 27 mer duplexeswith 6FAM-modified ends were less potent than the unmodified duplexes inreducing EGFP expression. Duplexes 1, 2 and 4, which showed partialcleavage with recombinant Dicer, had three- to six-fold reduced RNAiactivity. Duplex 5, which showed no cleavage with recombinant Dicer, hadminimal RNAi activity, establishing a direct correlation between therelative effectiveness of in vitro cleavage by Dicer and RNAi in cellculture.

Example 5 In vivo Processing by Dicer

This example confirms that the 27 mer dsRNA are processed in vivo byDicer.

Assay for Intracellular Processing of 27 Mer RNAs

RNA duplexes were transfected as described above into HEK293 cells in asix-well plate at 10 nM. After 14 h, total RNA was prepared as describedbelow. First, 20 μg of total RNA was heated for 10 min at 75° C. andmixed with ³²P 5′-end-labeled oligonucleotide probe(5′-ACCCTGAAGTTCATCTGCACC-3; SEQ ID NO:73) and hybridized in 150 mMNaCl, 50 mM Tris-HCl, pH. 7.4, 1 mM EDTA) at 24° C. Samples were loadedon 7.5% nondenaturing polyacrylamide gel and separated at 200 V for 3 hat 4° C., and the gel was exposed to X-ray film. ³²P-end-labeled 27 merand 21 mer duplex RNA oligos were used as size standards.

Results

To confirm that the 27 mer dsRNAs are processed by Dicer in vivo, wetransfected HEK293 cells with 10 nM of the duplex 3 (unmodified) orduplex 5 (both 3′ ends modified with 6FAM). After 14 h, total RNA wasisolated and hybridized with a ³²P end labeled 21 mer probe oligo (Sstrand). RNA was separated by nondenaturing PAGE and visualized byautoradiography (FIG. 2E). Similar to the results seen with in vitroDicer cleavage, in RNA prepared from cells transfected with duplex 3(unmodified 27 mer), a smaller species was observed migrating with a 21mer duplex marker, consistent with Dicer cleavage product. This 21 merspecies was not detected in RNA from cells transfected with duplex 5 (3′end-modified 27 mer).

Example 6 Potency of dsRNAs

This example examines the potency of potential cleavage products of the27 mer by Dicer.

Cleavage of a 27 mer by Dicer could result in a variety of distinct 21mers depending on where cleavage occurs; it is possible that one or amix of these possible 21 mers is significantly more potent than thespecific 21 mer that we used as our standard for comparison. To testthis possibility we synthesized seven different 21 mers that could bederived from the EGFPS1 27+0 duplex, walking in single-base steps alongthe antisense strand, using the traditional 21+2 design. These sevenduplexes were tested for RNAi activity in the HEK293 cell cotransfectionassay individually and as a pool (FIG. 3A). At concentrations of 50 or200 pM, neither the individual 21 mer duplexes nor the pooled set ofseven 21 mer duplexes showed activity comparable to the 27 mer duplex.In vitro Dicer cleavage of the 27 mers before transfection did notsignificantly enhance efficacy (FIG. 3B). As an additional control, wetransfected a mutated EGFP 27 mer duplex (Table 2), EGFPS1-27+0/mut)harboring four consecutive, centrally placed mismatched bases. Themismatches virtually eliminated any RNAi activity (FIG. 3B).

Example 7 Analysis of Dicer Cleavage Products

This example analyzed the in vitro Dicer cleavage products by massspectroscopy.

Electrospray-Ionization Liquid Chromatography Mass Spectroscopy

Electrospray-ionization liquid chromatography mass spectroscopy(ESI-LCMS) of duplex RNAs before and after treatment with Dicer weredone using an Oligo HTCS system (Novatia), which consisted ofThermoFinnigan TSQ7000, Xcalibur data system, ProMass data processingsoftware and Paradigm MS4 HPLC (Michrom BioResources). The liquidchromatography step used before injection into the mass spectrometer(LC-MS) removes most of the cations complexed with the nucleic acids;some sodium ions can remain bound to the RNA and are visualized as minor+22 or +44 species, reflecting the net mass gain seen with substitutionof sodium for hydrogen.

Results

The species that are actually produced by incubation of the EGFPS1 27+0duplex with recombinant Dicer in vitro were identified usingelectrospray ionization mass spectrometry (ESI MS) (FIGS. 4A and 4B).Calculated masses for each possible digestion product that could resultfrom in vitro Dicer cleavage are shown in Table 3. The ESI MS analysesof the in vitro cleavage products are consistent with the known activityof this enzyme.

TABLE 3 Molecular Weights of Possible 21 mer DuplexesDerived from the 27 mer Duplex by Dicer Processing Sequence (SEQ ID NO:)Name Mol Wt* 5′ AAGCUGACCCUGAAGUUCAUCUGCACC (41) EGFPS1-27 + 0 8552 3′UUCGACUGGGACUUCAAGUAGACGUGG (42) 8689 5′      ACCCUGAAGUUCAUCUGCACC (11) EGFPS1-21 + 2 (3) 6672** 3′    ACUGGGACUUCAAGUAGACGU (12) 6816** 5′      GACCCUGAAGUUCAUCUGCAC (13)EGFPS1-21 + 2 (4) 6712** 3′    GACUGGGACUUCAAGUAGACG (14) 6855 5′    UGACCCUGAAGUUCAUCUGCA (15) EGFPS1-21 + 2 (5) 6713** 3′  CGACUGGGACUUCAAGUAGAC (16) 6815** 5′    CUGACCCUGAAGUUCAUCUGC (17)EGFPS1-21 + 2 (6) 6689 3′  UCGACUGGGACUUCAAGUAGA (18) 6816** 5′  GCUGACCCUGAAGUUCAUCUG (19) EGFPS1-21 + 2 (7) 6729 3′UUCGACUGGGACUUCAAGUAG (20) 6793 *Molecular weight of 27 mer is theoriginal chemically synthesized duplex with hydroxyl ends. Calculatedweights of 21 mers assume 5′phosphate on each strand after DicerProcessing. **Indicates masses that were consistent with visualizedpeaks in FIG. 4B.

Example 8 Further Characterization of Inhibitory Properties of 27 MerdsRNA

To further characterize the inhibitory properties of the 27 mer dsRNA incells stably expressing the EGFP target, stably transfected NIH3T3 cellsexpressing EGFP were transfected with 21+2 and 27+0 dsRNA duplexes (bothat 5 nM). To obtain a quantitative estimate of the duration of genesuppression, we carried out a time-course experiment, observing EGFPexpression on days 2, 4, 6, 8 and 10 after transfection. Cell extractswere prepared and measured for EGFP fluorescence using a fluorometer(FIG. 5A). EGFP suppression lasted approximately 4 d using the 21+2siRNA, consistent with previous observations (Persengiev et al., 2004),whereas inhibition obtained with the 27+0 dsRNA persisted up to 10 d.This study shows the duration of the RNAi effect is at least about twiceas long with the 27-mer dsRNA of the invention as with 21-mers. A classof ‘hyperfunctional’ 21+2 siRNAs has been reported showing a similarextended duration of silencing (Reynolds et al., 2004); however, thesesequences are rare and difficult to find or predict. Use of the 27 merdsRNA design may permit longer, more potent RNAi to be achieved at agreater variety of target sites.

Example 9 Effect of 27 Mer dsRNA on Site Selection

A frequent problem in using RNAi as a tool to systematically inhibit theexpression of any gene is that not all target sites are equallysusceptible to suppression by siRNAs (Sherer and Rossi, 2004),necessitating complex design algorithms to predict effective sites(Reynolds et al., 2004; Ui-Tei et al., 2004; Amarzguioui and Prydz,2004). We therefore asked whether the increased potency of the 27 merdsRNA permits effective targeting at sites that are not active usingtraditional 21 mer siRNAs. Duplex RNAs were made having 21+2 and 27+0designs to two sites in EGFP (‘EGFP-S2’ and ‘EGFP-S3’) both previouslyshown to be refractory to RNAi using standard siRNAs (Kime and Rossi,2003).

Nucleic Acid Reagents

The reporter system employed EGFP as in SEQ ID NO:1 above. Site-2 (alsotermed bad site I) and Site-3 (also termed bad site 2) in EGFP weretargeted. RNA duplexes were synthesized and prepared as described inExample 1. Site-2 and Site 3 used for siRNA targeting in EGFP for thisexample were:

(SEQ ID NO: 74) SITE 2: 5′ UGAAGCAGCACGACUUCUUCAAGUCCGCCAUG 3′ and(SEQ ID NO: 75) SITE 3: 5′ UGAAGUUCGAGGGCGACACCCUGGUGAACCGCAU 3′.

RNA duplexes targeting EGFP Site-2 and EGFP Site-3 are summarized inTable 4.

TABLE 4 Summary of Oligonucleotide Reagents, EGFP Site-2 Sequence NameSEQ ID NO:  5′ UGAAGCAGCACGACUUCUUCAAGUCCGCCAUG 3′ EGFP Site-2SEQ ID NO: 74 5′     GCAGCACGACUUCUUCAAGUU EGFPS2-21 + 2 SEQ ID NO: 763′   UUCGUCGUGCUGAAGAAGUUC SEQ ID NO: 77 5′  AAGCAGCACGACUUCUUCAAGUCCGCC EGFPS2-27 + 0 SEQ ID NO: 78 3′  UUCGUCGUGCUGAAGAAGUUCAGGCGG SEQ ID NO: 79 5′UGAAGUUCGAGGGCGACACCCUGGUGAACCGCAU 3′ EGFP Site-3 SEQ ID NO: 75 5′    GUUCGAGGGCGACACCCUGUU EGFPS3-21 + 2 SEQ ID NO: 80 3′  UUCAAGCUCCCGCUCUGGGAC SEQ ID NO: 81 5′     GUUCGAGGGCGACACCCUGGUGAACUU EGFPS3-27 + 0 UU SEQ ID NO: 82 3′   UUCAAGCUCCCGCUCUGGGACCACUUGGCSEQ ID NO: 83

Results

The duplexes were transfected into HEK293 cells using the cotransfectionassay format (FIG. 5B) at 1 nM and 10 nM. At these doses, standard 21+2siRNAs were ineffective at both sites, whereas the 27 mer dsRNAs reducedEGFP expression by 80-90% at EGFP-S2 and by 50% (1 nM) and 80% (10 nM)at EGFP-S3. Despite the increased potency of Dicer substrate dsRNAs,empirical testing of target sites is still useful to find the mostsensitive targets for RNAi. In this regard, it is important that Dicerproducts of some 27 mers generated poorly functional siRNAs. By betterunderstanding Dicer substrate preferences, it should be possible todesign substrate RNAs that will generate the desired 21 mers. We haveobserved that a two-base 3′ overhang on only one end of the 27 mer willpreferentially guide Dicer to cleave 21-23 nt upstream of the two-base3′ overhang.

This example demonstrates that dsRNAs of the invention can efficientlytarget sites within the EGFP gene that were previously considered poortargets by previously known methods. Use of the method of the inventionwill therefore simplify site selection and design criteria for RNAi.This example also shows that the intentional placement of mismatches atthe 3′-terminus of the sense strand increases the potency of the 27 merduplex.

Example 10 Effect of 27 Mer dsRNA on Other Genes

To ensure that the increased potency of the 27 mer dsRNAs was not anartifact of targeting a reporter construct, we targeted two endogenoustranscripts, human hnRNP H mRNA (Markovtsov et al., 2000) and mRNAencoding the La protein (Wolin and Cedervall, 2002).

RNAi Assays Against hnRNP H and La

HEK293 cells were plated to 30% confluency in a six-well plate. The nextday, the cells were transfected with the indicated amount of dsRNA, andthe medium was changed on the following day. The cells were harvested in300 μl PBS 72 h after transfection. Extracts were prepared as describedabove for the EGFP assays. For western blots, 2 μl of cell extract wasloaded on a 10% SDS-PAGE gel. Endogenous hnRNP H was detected using arabbit polyclonal anti-hnRNP H antibody (Markovtsov et al., 2000) andanti-rabbit antibody conjugated to alkaline phosphatase (Sigma). β-Actinwas detected with a mouse-derived anti-actin antibody (Sigma) andanti-mouse antibody conjugated to alkaline phosphatase (Sigma). Fornorthern blot analyses, harvested cells were mixed with RNA STAT-60(Tel-Test B) and total RNA was extracted according to the manufacturer'sprotocol. RNA was electrophoresed in a 6% denaturing polyacrylamide gel,transferred to a nylon membrane and probed with ³²P-end-labeled oligos(La, 5′-CCAAAGGTACCCAGCCTTCATCCAGTT-3′ (SEQ ID NO:84); β-actin,5′-GTGAGGATGCCTCTCTTGCTCTGGGCCTCG-3′ (SEQ ID NO:85)). Hybridizationswere carried out in 10 ml of hybridization solution (1 ml 50×Denhardt's,3 ml 20×SSPE, 0.5 ml 10% SDS) for 3 h at 37° C. After hybridization, theblot was washed three times with 2×SSPE at 37° C.

Nucleic Acid Reagents

The coding sequence of Homo sapiens heterogeneous nuclearribonucleoprotein H (hnRPH) mRNA (Genbank accession No. NM_(—)005520) isshown in Table 5. The ATG start codon and TAA stop codons arehighlighted in bold font and site target by siRNA reagents isunderscored.

TABLE 5 Nucleotide Sequence of HNRPH (SEQ ID NO: 86)ttttttttttcgtcttagccacgcagaagtcgcgtgtotagtttgtttcgacgccggaccgcgtaagagacgatgatgttgggcacggaaggtggagagggattcgtggtgaaggtccggggcttgccctggtcttgctcggccgatgaagtgcagaggtttttttctgactgcaaaattcaaaatggggctcaaggtattcgtttcatctacaccagagaaggcagaccaagtggcgaggcttttgttgaacttgaatcagaagatgaagtcaaattggccctgaaaaaagacagagaaactatgggacacagatatgttgaagtattcaagtcaaacaacgttgaaatggattgggtgttgaagcatactggtccaaatagtcctgacacggccaatgatggctttgtacggcttagaggacttccctttggatgtagcaaggaagaaattgttcagttcttctcagggttggaaatcgtgccaaatgggataacattgccggtggacttccaggggaggagtacgggggaggccttcgtgcagtttgcttcacaggaaatagctgaaaaggctctaaagaaacacaaggaaagaatagggcacaggtatattgaaatctttaagagcagtagagctgaagttagaactcattatgatccaccacgaaagcttatggccatgcagcggccaggtccttatgacagacctggggctggtagagggtataacagcattggcagaggagctggctttgagaggatgaggcgtggtgcttatggtggaggctatggaggctatgatgattacaatggctataatgatggctatggatttgggtcagatagatttggaagagacctcaattactgtttttcaggaatgtctgatcacagatacggggatggtggctctactttccagagcacaacaggacactgtgtacacatgcggggattaccttacagagctactgagaatgacatttataattttttttcaccgctcaaccctgtgagagtacacattgaaattggtcctgatggcagagtaactggtgaagcagatgtcgagttcgcaactcatgaagatgctgtggcagctatgtcaaaagacaaagcaaatatgcaacacagatatgtagaactcttcttgaattctacagcaggagcaagcggtggtgcttacgaacacagatatgtagaactcttcttgaattctacagcaggagcaagcggtggtgcttatggtagccaaatgatgggaggcatgggcttgtcaaaccagtccagctacgggggcccagccagccagcagctgagtgggggttacggaggcggctacggtggccagagcagcatgagtggatacgaccaagttttacaggaaaactccagtgattttcaatcaaacattgcataggtaaccaaggagcagtgaacagcagctactacagtagtggaagccgtgcatctatgggcgtgaacggaatgggagggttgtctagcatgtccagtatgagtggtggatggggaatgtaattgatcgatcctgatcactgactcttggtcaacctttttttttttttttttttctttaagaaaacttcagtttaacagtttctgcaatacaagcttgtgatttatgcttactctaagtggaaatcaggattgttatgaagacttaaggcccagtatttttgaatacaatactcatctaggatgtaacagtgaagctgagtaaactataactgttaaacttaagttccagcttttctcaagttagttataggatgtacttaagcagtaagcgtatttaggtaaaagcagttgaattatgttaaatgttgccctttgccacgttaaattgaacactgttttggatgcatgttgaaagacatgcttttattttttttgtaaaacaatataggagctgtgtctactattaaaagtgaaacattttggcatgtttgttaattctagtttcatttaataacctgtaaggcacgtaagtttaagctttttttttttttaagttaatgggaaaaatttgagacgcaataccaatacttaggattttggtcttggtgtttgtatgaaattctgaggccttgatttaaatctttcattgtattgtgatttccttttaggtatattgcgctaagtgaaacttgtcaaataaatcctccttttaaaaactgc

The coding sequence of the La protein mRNA (Genbank accession No.NM_(—)005520) is shown in Table 6. The ATG start codon and TAA stopcodons are highlighted in bold font and site target by siRNA reagents isunderscored.

TABLE 6 Nucleotide Sequence of La Protein (SEQ ID NO: 87)ccggcggcgctgggaggtggagtcgttgctgttgctgtttgtgagcctgtggcgcggcttctgtgggccggaaccttaaagatagccgtaatggctgaaaatggtgataatgaaaagatggctgccctggaggccaaaatctgtcatcaaattgagtattattttggcgacttcaatttgccacgggacaagtttctaaaggaacagataaaactggatgaaggctgggtacctttggagataatgataaaattcaacaggttgaaccgtctaacaacagactttaatgtaattgtggaagcattgagcaaatccaaggcagaactcatggaaatcagtgaagataaaactaaaatcagaaggtctccaagcaaacccctacctgaagtgactgatgagtataaaaatgatgtaaaaaacagatctgtttatattaaaggcttcccaactgatgcaactcttgatgacataaaagaatggttagaagataaaggtcaagtactaaatattcagatgagaagaacattgcataaagcatttaagggatcaatttttgttgtgtttgatagcattgaatctgctaagaaatttgtagagacccctggccagaagtacaaagaaacagacctgctaatacttttcaaggacgattactttgccaaaaaaaatgaagaaagaaaacaaaataaagtggaagctaaattaagagctaaacaggagcaagaagcaaaacaaaagttagaagaagatgctgaaatgaaatctctagaagaaaagattggatgcttgctgaaattttcgggtgatttagatgatcagacctgtagagaagatttacacatacttttctcaaatcatggtgaaataaaatggatagacttcgtcagaggagcaaaagaggggataattctatttaaagaaaaagccaaggaagcattgggtaaagccaaagatgcaaataatggtaacctacaattaaggaacaaagaagtgacttgggaagtactagaaggagaggtggaaaaagaagcactgaagaaaataatagaagaccaacaagaatccctaaacaaatggaagtcaaaaggtcgtagatttaaaggaaaaggaaagggtaataaagctgcccagcctgggtctggtaaaggaaaagtacagtttcagggcaagaaaacgaaatttgctagtgatgatgaacatgatgaacatgatgaaaatggtgcaactggacctgtgaaaagagcaagagaagaaacagacaaagaagaacctgcatccaaacaacagaaaacagaaaatggtgctggagaccagtagtttagtaaaccaattttttattcattttaaataggttttaaacgacttttgtttgoggggcttttaaaaggaaaaccgaattaggtccacttcaatgtccacctgtgagaaaggaaaaatttttttgttgtttaacttgtctttttgttatgcaaatgagatttctttgaatgtattgttctgtttgtgttatttcagatgattcaaatatcaaaaggaagattcttccattaaattgcctttgtaatatgagaatgtattagtacaaactaactaataaaatatatactatatgaaaagagc

RNA duplexes were synthesized and prepared as described in Example 1.RNA duplexes targeting HNRPH1 are summarized in Table 7.

TABLE 7 Summary of Oligonucleotide Reagents Sequence Name SEQ ID NO:  5′GUUGAACUUGAAUCAGAAGAUGAAGUCAAAUUGGC 3′ HNRPH1 Site-1 SEQ ID No: 88 5′   CUUGAAUCAGAAGAUGAAGUU HNRPH1-21 + 2 SEQ ID No: 89 3′ UUGAACUUAGUCUUCUACUUC SEQ ID No: 90 5′   AACUUGAAUCAGAAGAUGAAGUCAAAUHNRPH1-27 + 0 SEQ ID No: 91 3′   UUGAACUUAGUCUUCUACUUCAGUUUASEQ ID No: 92 5′ AUAAAACUGGAUGAAGGCUGGGUACCUUUGGAGAU 3′ La Site-1SEQ ID NO: 93 5′       CUGGAUGAAGGCUGGGUACUU La-21 + 2 SEQ ID NO: 94 3′    UUGACCUACUUCCGACCCAUG SEQ ID NO: 95 5′    AACUGGAUGAAGGCUGGGUACCUUUUU La-21 + 2 SEQ ID NO: 96 3′    UUGACCUACUUCCGACCCAUGGAAACC SEQ ID NO: 97

Results

RNA duplexes were synthesized to target randomly chosen sites in thehuman hnRNP H mRNA (analyzed by western blotting) and the mRNA encodingthe La protein (analyzed by northern blotting (FIGS. 5C and 5D). Forboth targets the 27 mer duplex was more potent than the 21 mer siRNAstargeting these messages.

Example 11 Sequence Specificity of 27 mer

As a test for the sequence specificity of the 27 mer dsRNA, a series of27+0 dsRNAs with one, two or three mismatches to the EGFP target mRNAwere synthesized and tested at concentrations of 0 nM, 1 nM and 200 pMin the cotransfection assay (FIG. 6). The sequences of the mutated 27+0dsRNAs are shown in Table 2. At 200 pM, each of the mismatched sequenceswas less potent than the wild-type 27 mer dsRNA; the triple mismatch 27mer dsRNA was completely ineffective at triggering RNAi at allconcentrations tested. Similar results were obtained using a 27 merdsRNA targeted to ‘site 2’ of EGFP.

Example 12 Lack of Interferon Response

This example demonstrates that the dsRNA duplexes of the invention donot activate the interferon response.

Interferon and PKR Assays

After transfection of 293 cells with 20 nM of each RNA as describedpreviously, medium was collected after 24 h and used for ELISA assays ofinterferon α and β as previously described (Kim et al., 2004). The PKRactivation assay was done as previously described (Gunnery and Mathews,1998). PKR in the lysate was first activated by co-incubation of theindicated RNAs and radiolabeled by its auto-kinase reaction. Theradiolabeled PKR was then immunoprecipitated for analysis. To determinethe activity of PKR in the cell lysate without prior activation, dsRNAwas omitted. The reaction was incubated at 30° C. for 20 min. PKR fromthe reaction was immunoprecipitated using polyclonal antibody. Thepolyclonal antibody was added to the reaction, which was then placed onice for 1 h, followed by addition of 50 μl of 10% protein A-Sepharose inIPP500 (10 mM Tris, pH 8, 500 mM NaCl, 0.1% Nonidet P-40). This mixturewas rocked for 30 min at 4° C. The protein A-Sepharose beads were washedwith 1 ml IPP100 buffer (10 mM Tris, pH 8, 100 mM NaCl, 0.1% NonidetP-40) five times. After the last wash, the beads were boiled in proteinsample buffer and loaded onto an SDS-polyacrylamide gel followed byautoradiography.

Results

A potential problem in the use of the longer dsRNAs is activation of PKRand induction of interferons (Manche et al., 1992). We thereforeassessed whether a transfected 27 mer dsRNA activates interferon-α (FIG.7A) or interferon-β (FIG. 7B). As a positive control for interferoninduction, we transfected a triphosphate-containing single-stranded RNAwhich potently activated interferon-α and -β, as reported previously(Kim et al., 2004). Neither cytokine was detected when either the 21+2siRNA or 27+0 dsRNA was used. We have extended this observation to twoother 27 mer sequences specific for the EGFP-S2 and EGFP-S3 sites. PKRactivation in cell lysates was also assayed as described previously(Gunnery and Mathews, 1998). The lysate was treated with the indicatedRNAs, followed by immunoprecipitation. The positive control long dsRNAelicited PKR activation, but none of the shorter RNAs activated PKR(FIG. 7C).

Example 13 Asymmetric 27 Mer Duplex Design and Base Modifications

Can Influence Dicing Patterns and Allow Intelligent Design of 27 mer

This examples demonstrates that multiple species are produced by Diceraction on the 27 mer and that design of the 27 mer and/or inclusion ofbase modifications can be employed to direct these degradation patterns,limit heterogeneity, and predict end products.

It was demonstrated in Example 6, FIGS. 3A and 3B that all of theindividual 21 mers that could be produced by Dicer action on the EGFPS127 mer duplex are less potent in suppressing EGFP than the 27 merduplex. Nevertheless, which 21 mers are produced can influence ultimatepotency. An electrospray mass spectrometry assay was used to determinewhich 21 mers are the actual products that result from enzymaticdigestion of a 27 mer substrate RNA by Dicer. More than one 21 mer canresult from Dicer digestion of the 27 mer. Dicing patterns can becontrolled, permitting intelligent design.

Electrospray Mass Spectrometry Assay of In Vitro Dicing Reactions

RNA duplexes (100 pmoles) were incubated in 20 μl of 20 mM Tris pH 8.0,200 mM NaCl, 2.5 mM MgCl₂ with 1 unit of recombinant human Dicer(Stratagene, La Jolla, Calif.) for 12-24 hours. Electrospray-ionizationliquid chromatography mass spectroscopy (ESI-LCMS) of duplex RNAs pre-and post-treatment with Dicer were done using an Oligo HTCS system(Novatia, Princeton, N.J.), which consisted of ThermoFinnigan TSQ7000,Xcalibur data system, ProMass data processing software and Paradigm MS4™HPLC (Michrom BioResources, Auburn, Calif.). The liquid chromatographystep employed before injection into the mass spectrometer (LC-MS)removes most of the cations complexed with the nucleic acids; somesodium ion can remain bound to the RNA and are visualized as minor +22or +44 species, which is the net mass gain seen with substitution ofsodium for hydrogen. Accuracy of this assay is around +/−2 Daltons foroligonucleotides of this size.

Results

The EGFPS1-27+0 duplex was digested with Dicer and mass spectra wereobtained pre-digestion (FIG. 8A) and post-digestion (FIG. 8B). Ingeneral, Dicer will cleave a 27 mer duplex into 21 mer length fragmentswith a 5′-phosphate. Lesser amounts of 22 mers are also usuallygenerated. Small amounts of 20 mer and 23 mers can also sometimes beobserved. By comparing observed masses with the starting sequence, itcan be deduced that 4 duplexes with 2-base 3′-overhangs and 5′-phosphatewere produced in this dicing reaction. These species represent two majorcleavage products, both of which resulted in 21 mer and 22 mer duplexes.Lower case “p” represents a phosphate group. Calculated masses for eachpossible digestion product that could result from in vitro Dicercleavage are shown in Table 8.

TABLE 8 Molecular Weights of Possible DuplexesDerived from the 27 mer Duplex by Dicer Processing Sequence (SEQ ID NO:)Name Mol Wt 5′ AACCUGACCCUGAAGUUCAUCUCCACC (41) EGFPS1-27 + 0 L 8552 3′UUCGACUGGGACUUCAAGUAGACGUGG (42) 8689 5′     pACCCUGAAGUUCAUCUCCACC (11) EGFPS1-21 + 2(3) 6672 3′    ACUGGGACUUCAAGUAGACCUp (12) 6816 5′     pGACCCUGAAGUUCAUCUGCACC (98)EGFPS1-22 + 2(3) 7017 3′    GACUGGGACUUCAAGUAGACCUp (99) 7161 5′   pUGACCCUGAAGUUCAUCUCCA (15) EGFPS1-21 + 2(5) 6713 3′  CGACUGGGACUUCAAGUAGACp (16) 6815 5′   pCUGACCCUGAAGUUCAUCUCCA (100)EGFPS1-22 + 2(5) 7018 3′  UCGACUGGGACUUCAAGUAGACp (101) 7121

It was demonstrated in Example 2, FIGS. 1A-1C that blunt duplexes orduplexes with 5′-overhangs can show similar or better potency thanduplexes with 3′-overhangs. In these studies, both ends of the duplexwere symmetric (i.e., both ends were blunt, both ends were 3′-overhang,or both ends were 5′-overhang). In similar studies, it was found that ablunt 27 mer duplex with two bases mismatch at one end had higherpotency than the symmetric blunt, 3′-overhang, or 5′-overhang speciestested. The blunt duplex with 2-base mismatch on one end might mimic thebehavior of an asymmetric duplex with a 2-base 3′-overhang on one endand blunt on the other end. Asymmetric 27 mer duplexes of this kind weretested and found to have increased potency and fewer diced products(less heterogeneity) and resulted in a more predictable pattern.

The double-stranded RNA binding domains of many enzymes oftenspecifically recognize RNA and not DNA or some modified RNAs. Insertionof DNA residues into the ends of blunt 27 mer duplexes was studied.Asymmetric designs with DNA residues in the 3′-end of one strandresulted in fewer diced products (less heterogeneity) and generallyproduced a predicable pattern which was opposite that of the asymmetric3′-overhang designs.

The EGFPS1-27+0 duplex was modified to have an asymmetric 3′-overhang onone side and 2 DNA bases at the 3′-end of the other side. A 5-phosphatewas placed on the recessed strand at the 3-overhang to mimic dicerprocessing. The final duplex of this design is show below aligned withthe original blunt 27 mer. Lower case “t” represents DNA dT base andlower case “p” represents a phosphate group. Calculated masses are shownin Table 9.

TABLE 9 Molecular Weights of Duplexes Sequence (SEQ ID NO:) Name Mol Wt5′ AAGCUGACCCUGAAGUUCAUCUGCACC (41) EGFPS1-27 + 0 L 8552 3′UUCGACUGGGACUUCAAGUAGACGUGG (42) 8689 5′AAGCUGACCCUGAAGUUCAUCUGCACC (41) EGFPS1-27/25 L 8552 3′ttCGACUGGGACUUCAAGUAGACCUp (102) 8075

Mass spectra were obtained pre-digestion (FIG. 8C) and post-digestion(FIG. 8D). Analysis of the dicing products made from the modifiedasymmetric duplex was much simpler than for the symmetric duplex(compare FIGS. 8B with 8D). A single cleavage product was observed whichwas mostly 21 mer duplex with small amounts of 22 mer detectable. Lowercase “t” represents DNA dT base and lower case “p” represents aphosphate group. Calculated masses are shown in Table 10.

TABLE 10 Molecular Weights of Possible DuplexesDerived from the 27 mer/25 mer Duplex by Dicer ProcessingSequence(SEQ ID NO:) Name Mol Wt 5′ AAGCUGACCCUGAAGUUCAUCUGCACC (41)EGFPS1-27/25 L 8552 3′ ttCGACUGGGACUUCAAGUAGACCUp (102) 8075 5′     pACCCUGAAGUUCAUCUGCACC (11) EGFPS1-21 + 2(3) 6672 3′    ACUGGGACUUCAAGUAGACCUp (12) 6816 3′    GACUGGGACUUCAAGUAGACCUp (99)7161

Use of asymmetric design with a single 2-base 3′-overhang and selectiveincorporation of DNA residues simplifies the dicing reaction to yield asingle major cleavage product. This design additionally permitsprediction of the dicing pattern and allows for intelligent design of 27mers such that specific, desired 21 mers can be produced from dicing. Asdemonstration, a second 27 mer duplex, EGFPS1-27-R was studied whichoverlaps the EGFPS1-27-L sequence. Calculated masses are shown in Table11.

TABLE 11 Molecular Weights of Duplexes Sequence (SEQ ID NO:) Name Mol Wt5′ AAGCUGACCCUGAAGUUCAUCUGCACC (41) EGFPS1-27 + 0 L 8552 3′UUCGACUGGGACUUCAAGUAGACGUGG (42) 8689 5′    UGACCCUGAAGUUCAUCUGCACCACCG (103) EGFPS1-27 + 0 R 8528 3′    ACUGGGACUUCAAGUAGACGUGGUGGC (104) 8728

Mass spectra were obtained pre-digestion (FIG. 9A) and post-digestion(FIG. 9B). Analysis of the dicing products made from the EGFPS1-27+0 Rduplex showed a complex pattern similar to that seen with theEGFPS1-27+0 L duplex. Two major cleavage products were observed and both21 mer and 22 mer species were present. Very minor 20 mer species werealso seen. Lower case “p” represents a phosphate group. Calculatedmasses for each possible digestion product that could result from invitro Dicer cleavage are shown in Table 12.

TABLE 12 Molecular Weights of Possible DuplexesDerived from the 27 mer Duplex by Dicer Processing Sequence (SEQ ID NO:)Name Mol Wt 5′ UGACCCUGAAGUUCAUCUGCACCACCG (103) EGFPS1-27 + 0 R 8528 3′ACUGGGACUUCAAGUAGACGUGGUGGC (104) 8728 5′     pUGAAGUUCAUCUGCACCACCG (105) EGFPS1-21 (1) R 6712 5′   pCCUGAAGUUCAUCUGCACCACC (106) EGFPS1-22 (3)R 6977 3′  UGGGACUUCAAGUAGACGUGGUp (107) 7178 5′  pACCCUGAAGUUCAUCUGCACCACC (108)EGFPS1-27 + 0 R 6672 3′ ACUGGGACUUCAAGUAGACGUGGUp (109) 6816 3′ACUGGGACUUCAAGUAGACGUGp (110) EGFPS1-22 (5)R 7161

The EGFPS1-27+0-R duplex was converted to a DNA-modified, asymmetric25/27 mer duplex as shown below and this duplex was studied in the invitro dicing assay. Lower case “cg” represents DNA dCdG base and lowercase “p” represents a phosphate group. Calculated masses are shown inTable 13.

TABLE 13 Molecular Weights of Duplexes Sequence (SEQ ID NO:) Name Mol Wt5′ UGACCCUGAAGUUCAUCUGCACCACCG (103) EGFPS1-27 + 0 R 8528 3′ACUGGGACUUCAAGUAGACGUGGUGGC (104) 8728 5′ pACCCUGAAGUUCAUCUGCACCACcg (111) EGFPS1-25/27 R 7925 3′ACUGGGACUUCAAGUAGACGUGGUGGC (112) 8728

Mass spectra were obtained pre-digestion (FIG. 9C) and post-digestion(FIG. 9D). The DNA-modified asymmetric EGFPS1-25/25 R duplex showed aclean, single diced 21 mer species, as summarized in Table 14. Lowercase “p” represents a phosphate group.

TABLE 14 Molecular Weight of Possible DuplexDerived from the 25 mer/27 mer Duplex by Dicer ProcessingSequence (SEQ ID NO:) Name Mol Wt 5′  pACCCUGAAGUUCAUCUGCACCACcg (111)EGFPS1-25/27 R 7925 3′ ACUGGGACUUCAAGUAGACGUGGUGGC (112) 8728 5′ pACCCUGAAGUUCAUCUGCACC (11) EGFPS1-21 + 2(3) 6672 3′ACUGGGACUUCAAGUAGACCUp (12) 6816

If the results of FIGS. 8D and 9D are compared, it can be seen thatdigestion of the two different asymmetric duplexes EGFPS1-27/25 L andEGFPS1-25/27R result in formation of the same 21 mer species,EGFPS1-21(3). Lower case “t” or “cg” represents DNA bases and lower case“p” represents a phosphate group. Calculated masses are shown in Table15.

TABLE 15 Molecular Weights of Duplexes Sequence (SEQ ID NO:) Name Mol Wt5′ AAGCUGACCCUGAAGUUCAUCUGCACC (41) EGFPS1-27/25 L 8552 3′ttCGACUGGGACUUCAAGUAGACCUp (102) 8075 5′     pACCCUGAAGUUCAUCUGCACCACcg (111) EGFPS1-25/27 R 7925 3′    ACUGGGACUUCAAGUAGACGUGGUGGC (112) 8728 5′     pACCCUGAAGUUCAUCUGCACC (11) EGFPS1-21 + 2(3) 6672 3′    ACUGGGACUUCAAGUAGACCUp (12) 6816

Therefore use of the DNA-modified asymmetric duplex design as taught bythe invention reduces complexity of the dicing reaction for 27 mer RNAspecies and permits intelligent design of 27 mers for use in RNAi suchthat it is possible to specifically target a desired location. Cleavageof the substrate 27 mer by Dicer results in a unique, predictable 21 merwherein one end of the 21 mer coincides with the 3′-overhang end of thesubstrate 27 mer.

Example 14 Asymmetric 27 Mer Duplex Designs with Base Modifications CanImprove Potency over Blunt 27 Mers

This examples demonstrates that the new asymmetric RNA duplexes astaught by the invention, having a 2-base 3′-overhang on one side andblunt with 3-base 3′-DNA modification on the other side, can improvepotency over blunt 27 mer duplexes.

It was demonstrated in Example 13, FIGS. 8A-8D and 9A-9D, that use ofasymmetric duplexes can direct dicing and result in a single major 21mer cleavage product. Further, the 27/25 L and 25/27 R asymmetricduplexes both result in the same 21 mer duplex after dicing. Since thesame 21 mer duplex is produced from each of the two asymmetric 27 mers,it would be anticipated by one skilled in the art that these compoundsshould functionally have similar potency. It is shown in this examplethat this is not the case and that the 25/27 R design unexpectedly hasincreased potency relative to both the 27/25 L duplex and the blunt 27+0duplex.

RNA duplexes targeting EGFPS2 were co-transfected into HEK293 cells withan EGFP expression plasmid and assayed for EGFP activity after 24 hoursincubation according to methods described above. Transfected duplexesare shown in Table 16. Lower case “p” represents a phosphate group andlower case bases “cc” and “gt” represent DNA bases while uppercase basesrepresent RNA.

TABLE 16 Transfected Duplexes Sequence Name SEQ ID NO:  5′      GCAGCACGACUUCUUCAAGUU EGFPS2-21 + 2 SEQ ID NO: 76 3′    UUCGUCGUGCUGAAGAAGUUC SEQ ID NO: 77 5′    AAGCAGCACGACUUCUUCAAGUCCGCC EGFPS2-27 + 0 SEQ ID NO: 78 3′    UUCGUCGUGCUGAAGAAGUUCAGGCGG SEQ ID NO: 79 5′CAUGAAGCAGCACGACUUCUUCAAGUC EGFPS2-27/25L SEQ ID NO: 113 3′gtACUUCGUCGUGCUGAAGAAGUUCp SEQ ID NO: 114 5′     pGCAGCACGACUUCUUCAAGUCCGcc EGFPS2-25/27R SEQ ID NO: 115 3′    UUCGUCGUGCUGAAGAAGUUCAGGCGG SEQ ID NO: 79

The EGFPS2-21+2 and EGFPS2-27+0 RNA duplexes were employed previously inExample 9. The EGFPS2-27/25 L and EGFPS2-25/27 R duplexes are newasymmetric dsRNAs designed according to the present invention and targetthe same site, site II of EGFP. In the in vitro dicing electrospray massspectrometry assay as described in Example 13, both the EGFPS2-27/25 Land EGFPS2-25/27 R duplexes yield the same 21 mer product afterdigestion by Dicer, similar to the EGFPS2-21+2 duplex.

Transfection results are shown in FIG. 10A. As previously shown, theEGFPS2-21+2 duplexes had minimal activity in suppressing EGFP expressionwhile the 27+0 duplex showed significant inhibition. The 27/25 L duplexwas slightly less potent than the 27+0 duplex and the 25/27 R duplex wasmost potent. Based upon the teaching of prior art, this finding isunexpected, since both of the asymmetric duplexes produce the same 21mer species following dicing.

Similar transfections were done using the EGFPS1 duplexes 27/25 L and25/27 R. These duplexes produce the same 21 mer product, theEGFPS1-21(3) duplex, after dicing (Example 13). Transfected duplexes areshown in Table 17. Lower case “p” represents a phosphate group and lowercase bases “tt” represent DNA bases while uppercase bases represent RNA.

TABLE 17 Transfected Duplexes Sequence Name SEQ ID NO:  5′AAGCUGACCCUGAAGUUCAUCUGCACC EGFPS1-27/25 L SEQ ID NO: 41 3′ttCGACUGGGACUUCAAGUAGACGUp SEQ ID NO: 102 5′     pACCCUGAAGUUCAUCUGCACCACcg EGFPS1-25/27 R SEQ ID NO: 111 3′    ACUGGGACUUCAAGUAGACGUGGUGGC SEQ ID NO: 112

EGFP expression after transfection is shown in FIG. 10B. As before, the25/27 R duplex was significantly more potent than the 27/25 L duplex inreducing EGFP expression. In a similar experiment in which the bluntended 27 mer was compared with 25/27 R duplex and 27/25 L duplex, it wasfound that the dsRNAs had the following potencies:

-   -   25/27 R duplex>27/25 L duplex>27 mer.

27 mer duplex RNAs can show significantly higher potency than 21 merduplexes in suppressing targeted gene expression. The blunt 27 mers canresult in a variety of 21 mer species after dicing, so preciseperformance of a blunt 27 mer duplex cannot always be predicted. Thenovel asymmetric duplexes of the present invention wherein one side ofthe duplex has a 2-base 3′-overhang and the other side is blunt and hasbase modifications, such as DNA, at the 3′-end, force dicing to occur ina predictable way so that precise 21 mers result. These asymmetricduplexes, i.e., 27/25 L and 25/27 R, are each also more potent than the21 mers. The asymmetric 25/27 R design is the most potent embodiment ofthe present invention.

FIG. 11 is an illustration comparing the embodiments of the presentinvention. The target gene sequence is illustrated by SEQ ID NO:116. The“typical” parent 21 mer used as an siRNA molecule is shown aligned withthe target gene sequence. Aligned with the target gene and parent 21 mersequences is the L 27 mer v2.1 containing a 3′ overhang on the sensestrand and two DNA bases at the 3′ end of the antisense strand. Thediced cleavage product is also shown. This alignment illustrates theleft shift in designing these precursor RNAi molecules. Also alignedwith the target gene and parent 21 mer sequences is the R 27 mer v2.1containing a 3′ overhang on the antisense strand and two DNA bases atthe 3′ end of the sense strand. The diced cleavage product is alsoshown. This alignment illustrates the right shift in designing theseprecursor RNAi molecules.

Example 15 Determination of Effective Dose

This example demonstrates a method for determining an effective dose ofthe dsRNA of the invention in a mammal A therapeutically effectiveamount of a composition containing a sequence that encodes a dsRNA,(i.e., an effective dosage), is an amount that inhibits expression ofthe product of the target gene by at least 10 percent. Higherpercentages of inhibition, e.g., 20, 50, 90 95, 99 percent or higher maybe desirable in certain circumstances. Exemplary doses include milligramor microgram amounts of the molecule per kilogram of subject or sampleweight (e.g., about 1 microgram per kilogram to about 5 milligrams perkilogram, about 100 micrograms per kilogram to about 0.5 milligrams perkilogram, or about 1 microgram per kilogram to about 50 micrograms perkilogram). The compositions can be administered one or more times perweek for between about 1 to 10 weeks, e.g., between 2 to 8 weeks, orbetween about 3 to 7 weeks, or for about 4, 5, or 6 weeks, as deemednecessary by the attending physician. Treatment of a subject with atherapeutically effective amount of a composition can include a singletreatment or a series of treatments.

Appropriate doses of a particular dsRNA composition depend upon thepotency of the molecule with respect to the expression or activity to bemodulated. One or more of these molecules can be administered to ananimal, particularly a mammal, and especially humans, to modulateexpression or activity of one or more target genes. A physician may, forexample, prescribe a relatively low dose at first, subsequentlyincreasing the dose until an appropriate response is obtained. Inaddition, it is understood that the specific dose level for anyparticular subject will depend upon a variety of other factors includingthe severity of the disease, previous treatment regimen, other diseasespresent, off-target effects of the active agent, age, body weight,general health, gender, and diet of the patient, the time ofadministration, the route of administration, the rate of excretion, anydrug combination, and the degree of expression or activity to bemodulated.

The efficacy of treatment can be monitored by measuring the amount ofthe target gene mRNA (e.g. using real time PCR) or the amount of productencoded by the target gene such as by Western blot analysis. Inaddition, the attending physician can monitor the symptoms associatedwith the disease or disorder afflicting the patient and compare withthose symptoms recorded prior to the initiation of treatment.

It is clear from recent studies that the effects of RNAi are notentirely specific and that undesired ‘off-target’ effects can occur of amagnitude dependent on the concentration of siRNA (Persengiev et al.,2004). The new Dicer substrate dsRNA approach may facilitate use oflower concentrations of duplex RNA than are needed with traditional 21mer siRNAs. It is clear from published data that ‘off-target’ effectscan occur in certain cell lines using 21 mer siRNAs (Persengiev et al.,2004; Jackson et al., 2003), but these also can be minimized by usingreagents that have efficacy in the low to subnanomolar range (Persengievet al., 2004). To examine the potential for ‘off-target’ effects usingDicer substrate dsRNAs, we carried out microarray analyses comparing ansiRNA 21 mer with the 27 mer, each targeting EGFP site 1. NIH3T3 cellsthat stably express EGFP were transfected with concentrations of siRNAthat give effective target knockdowns (FIG. 2A, FIG. 7D). Total cellularRNAs were prepared from cells 24 and 48 h after transfection andanalyzed by hybridization to an oligonucleotide microarray as describedin FIG. 7D. Among the 16,282 mouse genes analyzed, only a small fractionshowed upregulation or downregulation more than twofold above or belowcontrol values (FIG. 7D). The 27 mer and 21 mer gave comparable resultsat their effective RNAi concentrations. There was an increase in thenumber of transcripts upregulated when the 27 mer was used at the higher25 nM concentration, but comparisons of the targets affected at 24versus 48 h and at 5 nM versus 25 nM showed no overlap. Rather thanspecific ‘off-target’ effects, these changes are more consistent withstatistical scatter among the 16,282 genes examined The same assay wasrepeated using the EGFP-S2 27+0 duplex RNA with comparable results.

Given the increase in potency of the 27 mer dsRNAs relative to 21+2siRNAs, it is of interest that this observation has not been previouslyreported. Although others have used dsRNAs of up to 27 bp for RNAistudies in mammalian cells (Bohula et al., 2003; Caplen et al., 2001),no differences in efficacy were reported as compared with traditional21+2 duplexes. This discrepancy between previous studies and our own maysimply be due to differences in the concentration of dsRNAs tested.“Good” sites for 21 mer siRNAs can have potencies in the nanomolar range(Reynolds et al., 2004). When ‘good’ sites are tested at highconcentrations of transfected RNA, differences between 21 mer siRNAs and27 mer dsRNAs will most likely be small and easily overlooked. Markeddifferences in potency are best shown by testing at low nanomolar orpicomolar concentrations, something that is not routinely done in mostlaboratories.

Thus far, the 27 mer dsRNA design has shown increased RNAi potencyrelative to 21+2 siRNAs at every site examined Within the set of 27 mersstudied here, however, a range of potencies is nevertheless seen betweendifferent target sites within the same gene (FIG. 3B). We have shownthat, even in the absence of fully optimized design rules, use of Dicersubstrate dsRNA approach can increase RNAi potency relative totraditional 21+2 siRNAs. Additionally, the use of 27 mer dsRNAs allowstargeting of some sites within a given sequence that are refractory tosuppression with traditional 21 mer siRNAs. Use of Dicer substratedsRNAs to trigger RNAi should result in enhanced efficacy and longerduration of RNAi at lower concentrations of RNA than are required for21+2 applications. Consistent with our results linking Dicer cleavage toenhanced RNAi efficacy, it has recently been shown that chemicallysynthesized hairpin RNAs that are substrates for Dicer are more potentinducers of RNAi than conventional siRNAs and, moreover, that a two-base3′ overhang directs Dicer cleavage (Siolas et al., 2005).

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Embodiments of this invention are described herein, including the bestmode known to the inventors for carrying out the invention. Variationsof those embodiments may become apparent to those of ordinary skill inthe art upon reading the foregoing description. The inventors expectskilled artisans to employ such variations as appropriate, and theinventors intend for the invention to be practiced otherwise than asspecifically described herein. Accordingly, this invention includes allmodifications and equivalents of the subject matter recited in theclaims appended hereto as permitted by applicable law. Moreover, anycombination of the above-described elements in all possible variationsthereof is encompassed by the invention unless otherwise indicatedherein or otherwise clearly contradicted by context.

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What is claimed is:
 1. A method of selecting a double stranded nucleicacid possessing increased potency in reducing expression of a targetgene in a mammalian expression system, said method comprising: (a)transfecting a first mammalian cell with a first double stranded nucleicacid at a transfecting concentration of 50 nanomolar or less, whereinsaid first double stranded nucleic acid comprises first and secondoligonucleotide strands, each strand comprising ribonucleotides andhaving a 5′ terminus and a 3′ terminus, wherein said first strand has alength which is at least 25 and at most 30 nucleotides, and wherein saidsecond oligonucleotide strand of said first double stranded nucleic acidhas a length which is at least 25 and at most 30 nucleotides andcomprises a sequence complementary to a target mRNA of a target gene andsaid first double stranded nucleic acid reduces target gene expressionwhen introduced into said first mammalian cell; (b) determining theamount of reduction in expression of a target gene in said firstmammalian cell relative to a control transfection; (c) transfecting asecond mammalian cell with a second double stranded nucleic acid at saidtransfecting concentration of 50 nanomolar or less, wherein said seconddouble stranded nucleic acid comprises first and second oligonucleotidestrands, each strand comprises ribonucleotides, has a 5′ terminus and a3′ terminus, consists of 21-23 nucleotides, and wherein said secondoliqonucleotide strand of said second double stranded nucleic acidcomprises said sequence complementary to said target mRNA; (d)determining the amount of reduction in expression of a target gene insaid second mammalian cell relative to a control transfection; (e)comparing said reduction in target gene expression of said step (b) tothe reduction in said target gene expression of step (d), whereby saidcomparing of said step (e) identifies at least a 10% greater reductionin said target gene expression by said first double stranded nucleicacid, as determined in step (b), relative to said second double strandednucleic acid, as determined in step (d), thereby permitting selection ofsaid first double stranded nucleic acid as a double stranded nucleicacid possessing increased potency.
 2. The method of claim 1 wherein saidfirst double stranded nucleic acid comprises a modified nucleotideselected from the group consisting of a deoxyribonucleotide, adideoxyribonucleotide, an acyclonucleotide, a 3′-deoxyadenosine(cordycepin), a 3′ azido-3′-deoxythymidine (AZT), a 2′,3′-dideoxyinosine(ddI), a 2′,3′-dideoxy-3′thiacytidine (3TC), a2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), a monophosphate nucleotideof 3′-azido-3′-deoxythymidine (AZT), a 2′,3′-dideoxy-3′-thiacytidine(3TC) and a monophosphate nucleotide of2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), a 4-thiouracil, a5-bromouracil, a 5-iodouracil, a 5-(3-aminoallyl)-uracil, a 2′-O-alkylribonucleotide, a 2′-O-methyl ribonucleotide, a 2′-amino ribonucleotide,a 2′-fluoro ribonucleotide, and a locked nucleic acid.
 3. The method ofclaim 1, wherein said first double stranded nucleic acid is cleaved byhuman Dicer in a mammalian cell so as to facilitate incorporation of acleaved second oligonucleotide strand into RISC.
 4. The method of claim1, wherein human Dicer cleavage of said first double-stranded nucleicacid preferentially results in a 21 to 22 nucleotide cleavage productcomprising said 3′ terminus of said second strand.
 5. The method ofclaim 1, wherein said second oligonucleotide strand of said first doublestranded nucleic acid is complementary to said target mRNA along atleast 19 nucleotides of said second oligonucleotide strand length. 6.The method of claim 1, wherein said transfecting concentration isselected from the group consisting of 1 nanomolar or less, 200 picomolaror less and 50 picomolar or less.
 7. The method of claim 1, wherein saidcomparing of said step (e) identifies a level of greater reduction insaid target gene expression by said first double stranded nucleic acidof at least 50%.
 8. The method of claim 1, wherein said first strand ofsaid first double stranded nucleic acid has a length which is at least26 nucleotides.
 9. The method of claim 1, wherein each of said first andsecond strands of said first double stranded nucleic acid has a lengthwhich is at least 26 nucleotides.
 10. The method of claim 1, whereineach of said first and said second strands of said first double strandednucleic acid consists of 26-30 nucleotides.
 11. The method of claim 1,wherein said second strand of said first double stranded nucleic acid is1-4 nucleotides longer at its 3′ terminus than said first strand of saidfirst double stranded nucleic acid.
 12. The method of claim 1, whereinsaid second strand of said first double stranded nucleic acid is 2nucleotides longer at its 3′ terminus than said first strand of saidfirst double stranded nucleic acid.
 13. The method of claim 1, whereinsaid first double stranded nucleic acid comprises a duplex region of atleast 25 nucleotides in length.
 14. The method of claim 1, wherein saidfirst double stranded nucleic acid comprises a duplex region of at least26 nucleotides in length.
 15. The method of claim 1, wherein said firstand second strands of said first double stranded nucleic acid comprisethe same number of nucleotide residues.
 16. The method of claim 1,wherein starting from the first nucleotide (position 1) at the 3′terminus of the first oligonucleotide strand of said first doublestranded nucleic acid, position 1, 2 and/or 3 is substituted with amodified nucleotide.
 17. The method of claim 16, wherein said modifiednucleotide residue of said 3′ terminus of said first strand of saidfirst double stranded nucleic acid is selected from the group consistingof a deoxyribonucleotide, an acyclonucleotide and a fluorescentmolecule.
 18. The method of claim 16, wherein position 1 of said 3′terminus of the first oligonucleotide strand of said first doublestranded nucleic acid is a deoxyribonucleotide.
 19. The method of claim16, wherein positions 1 and 2 of said 3′ terminus of the firstoligonucleotide strand of said first double stranded nucleic acid aredeoxyribonucleotides.
 20. The method of claim 1, wherein said 3′terminus of said first strand and said 5′ terminus of said second strandof said first double stranded nucleic acid form a blunt end.
 21. Themethod of claim 20, wherein said blunt end formed by said 3′ terminus ofsaid first strand and said 5′ terminus of said second strand of saidfirst double stranded nucleic acid is a base-paired blunt end.
 22. Themethod of claim 1, wherein said first strand of said first doublestranded nucleic acid is 25 nucleotides in length and said second strandof said first double stranded nucleic acid is 27 nucleotides in length.23. The method of claim 1, wherein said first double stranded nucleicacid is cleaved endogenously in a mammalian cell by Dicer.
 24. Themethod of claim 1, wherein said first double stranded nucleic acid iscleaved endogenously in a mammalian cell to produce a double strandednucleic acid of a length in the range of 19-23 nucleotides that reducestarget gene expression.
 25. The method of claim 1, wherein said secondstrand of said first double stranded nucleic acid is fully complementaryto the target mRNA.
 26. The method of claim 1, wherein said 5′ terminusof each of said first and said second strands of said first doublestranded nucleic acid comprises a 5′ phosphate.
 27. The method of claim1, wherein the relative length in nucleotide residues of said second andfirst strands of said first double stranded nucleic acid is selectedfrom the group consisting of: second strand 26-29 nucleotide residues inlength and said first strand 25 nucleotide residues in length, secondstrand 27-30 nucleotide residues in length and said first strand 26nucleotide residues in length, second strand 28-30 nucleotide residuesin length and first strand 27 nucleotide residues in length, secondstrand 29-30 nucleotide residues in length and first strand 28nucleotide residues in length, and second strand 30 nucleotide residuesin length and first strand 29 nucleotide residues in length.
 28. Themethod of claim 1, wherein said 3′ terminus of said first strand andsaid 5′ terminus of said second strand of said first double strandednucleic acid are joined by a chemical linker.
 29. The method of claim 1,wherein a nucleotide of said second or first strand of said first doublestranded nucleic acid is substituted with a modified nucleotide thatdirects the orientation of Dicer cleavage.
 30. The method of claim 1,wherein said first double stranded nucleic acid comprises a phosphatebackbone modification selected from the group consisting of aphosphonate, a phosphorothioate and a phosphotriester.
 31. A method ofselecting a double stranded nucleic acid possessing increased potency inreducing expression of a target gene in a mammalian expression system,said method comprising: (a) transfecting a first mammalian cell with afirst double stranded nucleic acid at a transfecting concentration of 50nanomolar or less, wherein said first double stranded nucleic acidcomprises first and second oligonucleotide strands, each strandcomprising ribonucleotides and having a 5′ terminus and a 3′ terminus,wherein said first strand has a length which is at least 25 and at most30 nucleotides, wherein said second strand is at least one and at mostfour nucleotides longer at its 3′ terminus than said first strand andforms a blunt end at its 5′ terminus with said 3′ terminus of said firststrand, wherein said first double stranded nucleic acid comprises aduplex region of at least 25 nucleotides in length, and wherein saidsecond oligonucleotide strand comprises a sequence complementary to atarget mRNA of a target gene and said first double stranded nucleic acidreduces target gene expression when introduced into said first mammaliancell; (b) determining the amount of reduction in expression of a targetgene in said first mammalian cell relative to a control transfection;(c) transfecting a second mammalian cell with a second double strandednucleic acid at said transfecting concentration of 50 nanomolar or less,wherein said second double stranded nucleic acid comprises first andsecond oligonucleotide strands, each strand comprises ribonucleotides,has a 5′ terminus and a 3′ terminus, consists of 21-23 nucleotides, andwherein said second oligonucleotide strand of said second doublestranded nucleic acid comprises said sequence complementary to saidtarget mRNA; (d) determining the amount of reduction in expression of atarget gene in said second mammalian cell relative to a controltransfection; (e) comparing said reduction in target gene expression ofsaid step (b) to the reduction in said target gene expression of step(d), whereby said comparing of said step (e) identifies at least a 10%greater reduction in said target gene expression by said first doublestranded nucleic acid, as determined in step (b), relative to saidsecond double stranded nucleic acid, as determined in step (d), therebypermitting selection of said first double stranded nucleic acid as adouble stranded nucleic acid possessing increased potency.
 32. Themethod of claim 31, wherein starting from the first nucleotide(position 1) at the 3′ terminus of the first oligonucleotide strand,position 1, 2 and/or 3 is substituted with a modified nucleotide. 33.The method of claim 31, wherein said first double stranded nucleic acidis cleaved by human Dicer in a mammalian cell so as to facilitateincorporation of a cleaved second oligonucleotide strand into RISC. 34.The method of claim 31, wherein human Dicer cleavage of said firstdouble-stranded nucleic acid preferentially results in a 21 to 22nucleotide cleavage product comprising said 3′ terminus of said secondstrand.
 35. The method of claim 31, wherein said second oligonucleotidestrand of said first double stranded nucleic acid is complementary tosaid target mRNA along at least 19 nucleotides of said secondoligonucleotide strand length.
 36. The method of claim 31, wherein saidfirst double stranded nucleic acid comprises a modified nucleotideselected from the group consisting of a deoxyribonucleotide, adideoxyribonucleotide, an acyclonucleotide, a 3′-deoxyadenosine(cordycepin), a 3′ azido-3′-deoxythymidine (AZT), a 2′,3′-dideoxyinosine(ddI), a 2′,3′-dideoxy-3′ thiacytidine (3TC), a2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), a monophosphate nucleotideof 3′-azido-3′-deoxythymidine (AZT), a 2′,3′-dideoxy-3′-thiacytidine(3TC) and a monophosphate nucleotide of2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), a 4-thiouracil, a5-bromouracil, a 5-iodouracil, a 5-(3-aminoallyl)-uracil, a 2′-O-alkylribonucleotide, a 2′-O-methyl ribonucleotide, a 2′-amino ribonucleotide,a 2′-fluoro ribonucleotide, and a locked nucleic acid.
 37. The method ofclaim 31, wherein said first double stranded nucleic acid comprises aphosphate backbone modification selected from the group consisting of aphosphonate, a phosphorothioate and a phosphotriester.
 38. A method ofselecting a double stranded nucleic acid possessing increased potency inreducing expression of a target gene in a mammalian expression system,said method comprising: (a) transfecting a first mammalian cell with afirst double stranded nucleic acid at a transfecting concentration of 50nanomolar or less, wherein said first double stranded nucleic acidcomprises first and second oligonucleotide strands, each strandcomprising ribonucleotides and having a 5′ terminus and a 3′ terminus,wherein said first strand has a length which is at least 25 and at most29 nucleotides and said second strand has a length which is at most 30nucleotides, wherein said 3′ terminus of said first strand and said 5′terminus of said second strand form a blunt end and said second strandis at least one and at most four nucleotides longer at its 3′ terminusthan said first strand, and wherein said second oligonucleotide strandcomprises a sequence complementary to a target mRNA of a target gene andsaid first double stranded nucleic acid reduces target gene expressionwhen introduced into said first mammalian cell (b) determining theamount of reduction in expression of a target gene in said firstmammalian cell relative to a control transfection; (c) transfecting asecond mammalian cell with a second double stranded nucleic acid at saidtransfecting concentration of 50 nanomolar or less, wherein said seconddouble stranded nucleic acid comprises first and second oliqonucleotidestrands, each strand comprises ribonucleotides, has a 5′ terminus and a3′ terminus, consists of 21-23 nucleotides, and wherein said secondoligonucleotide strand of said second double stranded nucleic acidcomprises said sequence complementary to said target mRNA; (d)determining the amount of reduction in expression of a target gene insaid second mammalian cell relative to a control transfection; (e)comparing said reduction in target gene expression of said step (b) tothe reduction in said target gene expression of step (d), whereby saidcomparing of said step (e) identifies at least a 10% greater reductionin said target gene expression by said first double stranded nucleicacid, as determined in step (b), relative to said second double strandednucleic acid, as determined in step (d), thereby permitting selection ofsaid first double stranded nucleic acid as a double stranded nucleicacid possessing increased potency.
 39. The method of claim 38, whereinstarting from the first nucleotide (position 1) at the 3′ terminus ofthe first oligonucleotide strand, position 1, 2 and/or 3 is substitutedwith a modified nucleotide.
 40. The method of claim 38, wherein saidfirst double stranded nucleic acid is cleaved by human Dicer in amammalian cell so as to facilitate incorporation of a cleaved secondoligonucleotide strand into RISC.
 41. The method of claim 38, whereinhuman Dicer cleavage of said first double-stranded nucleic acidpreferentially results in a 21 to 22 nucleotide cleavage productcomprising said 3′ terminus of said second strand.
 42. The method ofclaim 38, wherein said second oligonucleotide strand of said firstdouble stranded nucleic acid is complementary to said target mRNA alongat least 19 nucleotides of said second oligonucleotide strand length.43. The method of claim 38, wherein said first double stranded nucleicacid comprises a modified nucleotide selected from the group consistingof a deoxyribonucleotide, a dideoxyribonucleotide, an acyclonucleotide,a 3′-deoxyadenosine (cordycepin), a 3′ azido-3′-deoxythymidine (AZT), a2′,3′-dideoxyinosine (ddI), a 2′,3′-dideoxy-3′ thiacytidine (3TC), a2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), a monophosphate nucleotideof 3′-azido-3′-deoxythymidine (AZT), a 2′,3′-dideoxy-3′-thiacytidine(3TC) and a monophosphate nucleotide of2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), a 4-thiouracil, a5-bromouracil, a 5-iodouracil, a 5-(3-aminoallyl)-uracil, a 2′-O-alkylribonucleotide, a 2′-O-methyl ribonucleotide, a 2′-amino ribonucleotide,a 2′-fluoro ribonucleotide, and a locked nucleic acid.
 44. The method ofclaim 38, wherein said first double stranded nucleic acid comprises aphosphate backbone modification selected from the group consisting of aphosphonate, a phosphorothioate and a phosphotriester.
 45. The method ofclaim 1, wherein said transfecting concentration is selected from thegroup consisting of 10 nM or less, 1 nM or less, 200 pM or less and 50pM or less.
 46. The method of claim 1, wherein said transfectingconcentration is selected from the group consisting of 10 nM, 5 nM, 2.5nM, 1 nM, 200 pM and 50 pM.
 47. The method of claim 1, wherein saidfirst and said second mammalian cells are HEK 293 cells.
 48. The methodof claim 1, wherein said first and said second mammalian cells areNIH3T3 cells.
 49. The method of claim 31, wherein said transfectingconcentration is selected from the group consisting of 10 nM or less, 1nM or less, 200 pM or less and 50 pM or less.
 50. The method of claim31, wherein said transfecting concentration is selected from the groupconsisting of 10 nM, 5 nM, 2.5 nM, 1 nM, 200 pM and 50 pM.
 51. Themethod of claim 31, wherein said first and said second mammalian cellsare HEK 293 cells.
 52. The method of claim 31, wherein said first andsaid second mammalian cells are NIH3T3 cells.
 53. The method of claim38, wherein said transfecting concentration is selected from the groupconsisting of 10 nM or less, 1 nM or less, 200 pM or less and 50 pM orless.
 54. The method of claim 38, wherein said transfectingconcentration is selected from the group consisting of 10 nM, 5 nM, 2.5nM, 1 nM, 200 pM and 50 pM.
 55. The method of claim 38, wherein saidfirst and said second mammalian cells are HEK 293 cells.
 56. The methodof claim 38, wherein said first and said second mammalian cells areNIH3T3 cells.
 57. The method of claim 1, wherein said steps (b) and (d)comprise providing a biological sample comprising a product of saidtarget gene expression and quantifying said product.
 58. The method ofclaim 57, wherein said target gene expression product comprises RNA. 59.The method of claim 1, wherein said step (b) comprises preparing anextract of said first mammalian cell and quantifying the amount of atarget gene expression product in said extract.
 60. The method of claim59, wherein said target gene expression product comprises RNA.
 61. Themethod of claim 1, wherein said step (d) comprises preparing an extractof said second mammalian cell and quantifying the amount of a targetgene expression product in said extract.
 62. The method of claim 61,wherein said target gene expression product comprises RNA.
 63. Themethod of claim 1, wherein said step (e) comprises comparing averagedgene expression product level values of step (b) with averaged geneexpression product level values of step (d).
 64. The method of claim 1,wherein said step (e) comprises comparing gene expression product levelsrepresented as histograms.
 65. The method of claim 31, wherein saidsteps (b) and (d) comprise providing a biological sample comprising aproduct of said target gene expression and quantifying said product. 66.The method of claim 65, wherein said target gene expression productcomprises RNA.
 67. The method of claim 31, wherein said step (b)comprises preparing an extract of said first mammalian cell andquantifying the amount of a target gene expression product in saidextract.
 68. The method of claim 67, wherein said target gene expressionproduct comprises RNA.
 69. The method of claim 31, wherein said step (d)comprises preparing an extract of said second mammalian cell andquantifying the amount of a target gene expression product in saidextract.
 70. The method of claim 69, wherein said target gene expressionproduct comprises RNA.
 71. The method of claim 31, wherein said step (e)comprises comparing averaged gene expression product level values ofstep (b) with averaged gene expression product level values of step (d).72. The method of claim 31, wherein said step (e) comprises comparinggene expression product levels represented as histograms.
 73. The methodof claim 38, wherein said steps (b) and (d) comprise providing abiological sample comprising a product of said target gene expressionand quantifying said product.
 74. The method of claim 73, wherein saidtarget gene expression product comprises RNA.
 75. The method of claim38, wherein said step (b) comprises preparing an extract of said firstmammalian cell and quantifying the amount of a target gene expressionproduct in said extract.
 76. The method of claim 75, wherein said targetgene expression product comprises RNA.
 77. The method of claim 38,wherein said step (d) comprises preparing an extract of said secondmammalian cell and quantifying the amount of a target gene expressionproduct in said extract.
 78. The method of claim 77, wherein said targetgene expression product comprises RNA.
 79. The method of claim 38,wherein said step (e) comprises comparing averaged gene expressionproduct level values of step (b) with averaged gene expression productlevel values of step (d).
 80. The method of claim 38, wherein said step(e) comprises comparing gene expression product levels represented ashistograms.